Amid rising tensions with Iran, American officials have abruptly revoked the visas of dozens of Iranian professionals headed to a university reunion in Northern California this weekend, refusing them entry as they landed at several American airports.

The men and women had obtained 15-day visitor visas to attend the fourth global alumni reunion of Iran’s Sharif University of Technology, a prestigious institution known as the “MIT of Iran.”

While a handful successfully entered America, by the time the association festivities began at the Santa Clara Hyatt Regency late Friday, it appeared that visas of the bulk of alumni from Iran had been revoked under a 2002 national security law.

Organizers said they knew of about 40 barred from entering America in recent days. About a dozen of the visitors, some traveling with spouses and children, were detained at San Francisco International Airport on Thursday, and some were held overnight in what one described to a friend in a brief phone call as “jail conditions.”

A State Department spokeswoman said she could not discuss the cases because of confidentiality laws, but stressed that visa revocations in general are individual decisions and not politically motivated.

Individual revocations of visas have been common in the post-September 11 era. But immigration and human rights attorneys condemned the apparent en masse crackdown on the Sharif alumni as a shortsighted political move inspired by recent tensions over Iran’s nuclear program and links to Hezbollah.

“To punish Iranians who are potential allies of pro-democracy steps in order to somehow punish the Iranian government is just inane,” a Los Angeles-based human rights and constitutional law attorney, Peter Schey, said.

Conference organizers and immigration attorneys who scrambled unsuccessfully to gain access to the detainees said it was not clear how many had been stopped and turned back.

An indication of trouble came 10 days ago, when a 49-year-old electrical engineer, Kourosh Elahidoost, was turned back at Los Angeles International Airport. Organizers first believed his case to be isolated, but as dozens more alumni were turned away in Chicago, New York, Toronto, San Francisco, and in Europe, they realized it was systematic.

In a telephone interview Friday from Tehran, Mr. Elahidoost said he was told by consular officials in Vienna that his visa was revoked under an American law that bars the issuance of visas to nationals of Iran and four other countries regarded as “state sponsors of terrorism,” unless the person is deemed to be no threat to national security.

Mr. Elahidoost said he was depressed and befuddled over why he was barred from what was to be his first visit to America. He was held overnight at a detention center in Santa Ana.

“I have never been a political person in my whole life,” he said. Mr. Elahidoost is a board member of Parstableau, an Iranian firm that manufactures electrical switchboards. “I have never joined any political organization or the government. Never.”

Reunion organizers in California criticized America’s actions, saying they were targeting Iran’s best and brightest technocrats, many of them Western-educated, who could help ease the volatile relations between the countries. About 120 Iranians had received visas to attend the reunion, Fredun Hojabri, who is a San Diego retired professor and conference organizer who founded the alumni association in 2000, said.

“These are not revolutionaries or crazies, these are among the most educated elite in Iran,” a University of Southern California engineering professor who was planning to attend the reunion, Najmedin Meshkati, said. “If these guys posed a security risk, they shouldn’t have been issued a visa in the first place.

“This is not the way to win Iranian hearts and minds,” Mr. Meshkati said.

Mr. Hojabri said he was mystified as to why a few Iranians were admitted and others were not when all of them were issued valid American visas as recently as a week ago. He and other association members speculated that the actions were tied to rising American tensions with Iran over the nuclear issue and the current conflict in Lebanon between Israel and Hezbollah.

But the State Department spokeswoman dismissed any link. In general, she said, any decision to revoke a visa is made on an individual basis for very specific reasons outlined in the law. These include a criminal background, ties to terrorism, and failure to present evidence of an intention to return to the home country, she said.