An estimated 10,000 pro-democracy protesters gathered at the city's university on Monday, shouting "Death to dictatorship!"

Jim Muir reports from Tehran: "Most serious clashes of their kind"

As students began taking to the streets in other towns, Iran's reformist President Mohammad Khatami appealed for calm in Tehran and warned students to be wary of "provocations" from opponents of reform.

According to eyewitnesses, police beat the students with sticks and used tear gas to drive them back. Some reports said that hundreds of local residents and passers-by joined in the demonstrations.

Thousands of other students remain barricaded inside the university campus, whose gates have been shut. Flames from a large fire could be seen from outside the campus.

Witnesses also said a shot was fired outside the university's central mosque, sending thousands of demonstrators spilling on to the campus.

Ayatollah Khamenei has told supporters not to resort to violence

Earlier, angry students had shouted down attempts to read a message of condolence from Iran's supreme clerical leader, Ayatollah Ali Khamenei, for the death of a demonstrator last week.

Ayatollah Khamenei denounced the police raid that killed one student on Thursday as "unacceptable''. "This bitter incident hurt my heart," he was quoted as saying.

"In the Islamic system it is not acceptable at all to attack the house and shelter of a group, particularly overnight or at the time of congregation prayers," he added.

Ayatollah Khamenei is identified with Iran's hardline conservatives opposed to reformist President Muhammad Khatami who is supported by the students.

The Supreme Leader's remarks, quoted by the official IRNA news agency, are his first public reaction to the pro-democracy protests and follow allegations by the students that he was complicit in the police action.

Stone throwing

The protests continued on Monday despite a warning from Iran's National Security Council that demonstrations now require authorisation.

The BBC's Jon Leyne: "The future direction of Iran is at stake"

Protests have also erupted in several other cities in Iran: the national news agnency IRNA reported student demonstrations in Yazd in central Iran and in the western cities of Khorramabad and Hamadan, and in the north in Shahrud.

They are calling for the dismissal of Iran's hardline police chief, General Hedeyat Lotfian, and for control of law-enforcement agencies to be transferred from the Islamic authorities to President Khatami.

Reports said protesters threw stones at police who retaliated with batons.

Police detained

Two senior police officers have been dismissed and detained for ordering Thursday's raid.

President Khatami has come down on the side of the students

Iranian TV said General Muhammad Ahmadi and his unnamed deputy had been handed over to judiciary officials.

The Iranian news agency said on Monday that ''dozens of students'' had been injured and many others arrested in the recent unrest.

As the protests have grown and spread to cities outside Tehran they have become increasingly critical of Iran's Islamic authorities.

The BBC Middle East correspondent says the unprecedented demonstrations have given President Khatami's hardline opponents a glimpse of what might happen if they continue to obstruct peaceful reform.

More than 500 journalists working for pro-reform newspapers are planning a one-day strike against the closure of the Salam newspaper - which sparked the protests - on Tuesday.