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NEW YORK — His first impulse was to dismiss the ominous email as a prank, says a young Iranian-American named Koosha. It warned the 29-year-old engineering student that his relatives in Tehran would be harmed if he didn’t stop criticizing Iran on Facebook.

Two days later, his mom called. Security agents had arrested his father in his home in Tehran and threatened him by saying his son could no longer safely return to Iran.

“When they arrested my father, I realized the email was no joke,” said Koosha, who asked that his full name not be used.

Tehran’s leadership faces its biggest crisis since it first came to power in 1979, as Iranians at home and abroad attack its legitimacy in the wake of June’s allegedly rigged presidential vote. An opposition effort, the “Green Movement,” is gaining a global following of regular Iranians who say they never previously considered themselves activists.

The regime has been cracking down hard at home. And now, a Wall Street Journal investigation shows, it is extending that crackdown to Iranians abroad as well.

In recent months, Iran has been conducting a campaign of harassing and intimidating members of its diaspora world-wide — not just prominent dissidents — who criticize the regime, according to former Iranian lawmakers and former members of Iran’s elite security force, the Revolutionary Guard, with knowledge of the program.

Part of the effort involves tracking the Facebook, Twitter and YouTube activity of Iranians around the world, and identifying them at opposition protests abroad, these people say.

Interviews with roughly 90 ordinary Iranians abroad — college students, housewives, doctors, lawyers, businesspeople — in New York, London, Dubai, Sweden, Los Angeles and other places indicate that people who criticize Iran’s regime online or in public demonstrations are facing threats intended to silence them.

Although it wasn’t possible to independently verify their claims, interviewees provided consistently similar descriptions of harassment techniques world-wide. Most asked that their full names not be published.

Today’s crisis echoes the events of three decades ago, when Iran’s Islamic revolution first bloomed. Back then, Iranians around the world pooled their energy and money to help oust Iran’s monarch, the shah. This time, the global community is backing a similar effort, using new tools including Facebook and Twitter. YouTube videos providing step-by-step instructions for staging civil disobedience rack up thousands of views.

But now, unlike 30 years ago, Iran’s leadership is striking back across national borders.

Dozens of individuals in the U.S. and Europe who criticized Iran on Facebook or Twitter said their relatives back in Iran were questioned or temporarily detained because of their postings. About three dozen individuals interviewed said that, when traveling this summer back to Iran, they were questioned about whether they hold a foreign passport, whether they possess Facebook accounts and why they were visiting Iran. The questioning, they said, took place at passport control upon their arrival at Tehran’s Imam Khomeini International Airport.

Five interviewees who traveled to Iran in recent months said they were forced by police at Tehran’s airport to log in to their Facebook accounts. Several reported having their passports confiscated because of harsh criticism they had posted online about the way the Iranian government had handled its controversial elections earlier this year.

Before this past summer, “If anyone asked me, ‘Does the government threaten Iranians abroad or their families at home,’ I would say, ‘Not at all,'” says Nasrin Sotoudeh, a prominent lawyer inside Iran. “But now the cases are too many to count. Every day I get phone calls and visits from people who are being harassed and threatened” because of relatives’ activities abroad.

In November, the deputy commander of Iran’s armed forces, Gen. Massoud Jazayeri, wrote an editorial in the conservative newspaper Kayhan that “protesters inside and outside Iran have been identified and will be dealt with at the right time.”

In Germany, a national intelligence report indicates that Iranian intelligence operatives are monitoring about 900 critics of the Iranian regime within Germany. One German intelligence official, Manfred Murch, said last month that his staff has identified “Iranian intelligence agents” trying to intimidate protesters in Germany by videotaping them. A German foreign-ministry official said Germany rejected requests from Iran to restrict anti-Iranian protests there.

Mohammad Reza Bak Sahraei, a diplomat at Iran’s mission to the United Nations in New York, didn’t respond to written questions about Iran’s intelligence activities abroad. “The allegation that the Islamic Republic of Iran has created limitations and problems for Iranians who are visiting Iran from abroad is false,” Mr. Sahraei said.

In recent months, he said, “Many Iranians have returned to Iran and visited their family members. Until now we have no reports of any limitations being imposed on them. Representatives of Iran abroad are doing their utmost to facilitate traveling for Iranians to Iran.”

The crisis in Iran started with June’s controversial re-election of Iran’s president, Mahmoud Ahmadinejad. Claims of vote fraud spawned massive street protests, and a bloody crackdown.

The post-election violence has turned Iran’s relationship with overseas Iranians on its head. Previously, Iran generally enjoyed good relations with its diaspora. Most opposition movements were on the fringe — for instance, royalists calling for the shah’s return. But the violent suppression of street protests “showed people the true nature of Iran’s regime,” says Karim Sadjadpour, an Iran analyst for the Carnegie Endowment for International Peace.

There are approximately four million Iranians abroad. The U.S. is home to the largest number, totaling at least several hundred thousand. They rank among the nation’s best educated and most affluent immigrant groups.

At first, many protesters inside Iran and abroad simply wanted a vote recount. But after the violence, they began calling for a complete overhaul of Iran’s Islamic system, up to and including change that would remove Supreme Leader Ayatollah Ali Khamenei from power. Around the world, Iranians took to the streets to march in protest against the events in Iran.

Associated Press Iranian police in June chase protesters after the controversial election.

An Iranian engineer in his 30s who lives in a German-speaking area of Europe, and who attended protests there this year, described having his passport, cellphone and laptop confiscated when he later traveled to Tehran. He said he was called in for questioning several times, blindfolded, kicked and physically abused, and asked to hand over his email and Facebook passwords.

Interrogators showed him images of himself participating in protests in Europe, he said, and pressed him to identify other people in the images.

“I was very scared. My knees were trembling the whole time and I kept thinking, ‘How did this happen to me?'” he said recently. “I only went to a few demonstrations, and I don’t even live in Iran.”

He said he was told he was guilty of charges including attending antiregime protests abroad, participating in online activities on Facebook and Twitter that harmed Iran’s national security and leaving comments on opposition Web sites. He said he was given a choice: Face trial in Iran, or sign a document promising to act as an informant in Europe.

He says he signed the paper, took his passport and left Iran after a month. He says he has received follow-up emails and phone calls but hasn’t responded to them.

Other Iranians abroad report receiving email threats tied to their online activities. In Los Angeles in June, an Iranian-American graduate student named Hamid said he received an email that read in part: “Stop spreading lies about Iran on Facebook.” He said he received it after he changed his Facebook profile picture to a “V” symbol, for victory, dripping with blood to protest the Iran violence, along with a message about wanting to travel to Iran to support the opposition.

The email, written in Farsi, read in part, “We know your home address in Los Angeles. Watch out, we will come after you,” according to Hamid.

There is no way to identify the email’s anonymous sender, who signed it “Spider.” Other Iranians interviewed in the U.S. and Europe reported receiving similar emails in recent months. Some emails were signed “Spider,” they said, while others were signed “Revolutionary Hossein,” a possible reference to one of the most revered saints in Shiite Islam.

No matter how widespread, the worries are sowing panic in the overseas community. Concerns about the safety of friends and family are so prevalent among younger Iranians that a number have changed their surnames on Facebook to “Irani” (which means simply “from Iran”) to be harder to single out.

Omid Habibinia, a dissident Iranian who left Iran seven years ago for Europe, says he has always been harassed, but the pressure has grown this year. He claims Iranian security services early this year created a fake Facebook account for him and tried to “friend” people on his behalf and ask them questions. Other Iranian dissidents, along with some journalists, described similar experiences.

Officials at Facebook said the company often gets reports of fake profiles and will remove them after a review. A spokeswoman declined to comment on specific profiles that have been removed, including the one Mr. Habibinia described. She said deleted profiles no longer reside on Facebook’s servers, making it impossible to trace their origins. She said she wasn’t aware of complaints of harassment on Facebook at the hands of Iranian security services.

One 28-year-old physician who lives in Dubai said that in July he was asked to log on to his Facebook account by a security guard upon arrival in Tehran’s airport. At first, he says, he lied and said he didn’t have one. So the guard took him to a small room with a laptop and did a Google search for his name. His Facebook account turned up, he says, and his passport was confiscated.

After a month and several rounds of interrogations, he says, he was allowed to exit the country.

During Iran’s historic 1979 Islamic revolution, Iranians abroad played an instrumental role in transforming the movement from a fringe idea led by a frail cleric, Ayatollah Ruhollah Khomeini, into a global force that eventually toppled the monarchy of Shah Mohammad Reza Pahlavi. Iranians abroad flocked to Mr. Khomeini’s side, lending his movement language skills, money and, ultimately, global legitimacy.

In the current crisis, Iran is eager to prevent a similar scenario.

To cut communication between Iranians inside and outside the country, Iran slowed Internet speeds so that accessing an online email account could take close to a half-hour. It blocked access to Facebook, YouTube and Twitter. For a while, an automated message warned people making international phone calls not to give information to outsiders.

Tracking Internet crimes — from political dissent to pornography — has long been a priority of the regime. Iran’s local media openly report on Internet-monitoring centers inside the country’s judiciary and armed forces that are staffed with English-speaking, tech-savvy young people.

Late last month, at a military parade in Tehran, intelligence minister Heydar Moslehi announced the training of “senior Internet lieutenants” to confront Iran’s “virtual enemies online.” This month Iran announced a 12-member unit within the armed forces called the Internet Crime Unit to track individuals “spreading lies and insults” about the regime.

Iran’s elite security force, the Revolutionary Guard Corps, along with the intelligence ministry each have their own, separate Internet-monitoring units that track prominent political figures and activists, according to dissidents including Mohsen Sazegara, one of the original founders of the Revolutionary Guard who is now in exile in the U.S. After the June election crisis, these Internet-monitoring units expanded their work to include the online activity of Iranians abroad, these people say.

In the U.S., Koosha, the young engineering student whose father was briefly arrested in Tehran, says he was never politically active before. But this past summer, he said, he watched the turmoil in Iran and “I couldn’t just sit and do nothing, I felt too guilty.” He watched “people my age getting beaten and killed in the streets for expressing their opinion,” he said. “The least I could do was to show my solidarity.”

That’s when he took steps that attracted the unwelcome attention. He attended a few rallies organized by opposition supporters near where he lives in the U.S. And then, when a prominent human-rights lawyer was jailed in Iran, Koosha created an online petition.

After his father was detained, Koosha took down his petition. “I was terrified and furious,” he said. And he doesn’t talk politics anymore when he calls his parents in Tehran.

But he’s still finding ways to express his views. In September, he biked from Toronto to New York with his brother as part of the group Bicycling for Human Rights in Iran. “They want to control even Iranians who don’t live under their rule,” he says.

—Jeanne Whalen in London, David Crawford in Berlin and Christopher Rhoads in New York contributed to this article. Write to Farnaz Fassihi at farnaz.fassihi@wsj.com