By FARNAZ FASSIHI

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.

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Associated Press

Iranian artist Shirin Neshat, third from right, leads actors in expressing support for Iran's opposition movement at the Venice film festival in September.

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 Most asked that their full names not be published.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.