EHRAN, July 9 — Armed vigilantes arrested three student leaders this morning after a news conference at which they announced the cancellation of protests to mark the anniversary of the 1999 student protests.

The three students, Ali Moghtaderi, Arash Hashemi and Reza Amerinassab, were thrown into three separate cars by about 15 armed men. Mr. Moghtaderi's face was covered with blood, after having been shoved to the ground by the men.

They joined the 4,000 people authorities have said were arrested during 10 days of protest last month. Many of those arrested remain in jail.

The Tehran police later said the students' arrest had been carried out by the hard-line prosecutor's office, which has jailed scores of pro-democracy demonstrators.

The government banned any demonstrations to commemorate the unrest of 1999, in which one student was killed.

In addition, the authorities shut down universities this week, forced students to evacuate their dormitories, and stationed paramilitary forces at mosques near a major student dormitory in Tehran.

Nonetheless, thousands responded to calls from opposition television and radio stations based outside the country and drove in their cars through the city this evening.

The protesters clashed with vigilante forces outside the Tehran university and the police fired tear gas to disperse them.

At the morning news conference, Mr. Amerinassab said the day's protest, to have taken place outside the United Nations office here, had been canceled on the advice of supportive members of Parliament.

But he said the decision did not mean they were withdrawing their demands for the release of fellow students.

"The student movement has been torn apart by arrests and pressure, but we still wish to pursue our demands through civil channels," he said.