Iranian exile TV broadcasts urged people to demonstrate
More than 1,000 people have clashed with riot police in the Iranian capital Tehran, in the first major protest against the Islamic regime for more than six months.
The action began as a demonstration by students against plans to privatise some universities, but they were joined by hundreds more people chanting slogans against the powerful Muslim clerics.
Several shop windows were broken and a few motorcycles burnt as baton-wielding police dispersed the protesters.
The crowd swelled after calls to participate were broadcast by US-based satellite TV channels.
For more than four hours the night was filled with bursts of noise as people set off firecrackers or sounded the horns from their cars.
Protesters called for political prisoners to be freed and for President Mohammad Khatami to resign.
Eyewitnesses say the protest turned violent when riot police tried to disperse the demonstration, beating those that stood in their way.
The protest may be the first of many demonstrations to commemorate student riots which shook Iran in July 1999, the BBC's Miranda Eeles reports from Tehran.
Police have not faced such hostile crowds for months
During those riots students clashed violently with police for three days after a raid on a university dormitory which left at least one person dead.
The authorities have called on students not to repeat such incidents.
The 1999 crackdown marked the worst anti-government clashes since the Iranian revolution in 1979.
Earlier on Tuesday, Iran's biggest student organisation criticised the country's supreme leader, Ayatollah Khamenei, urging him to choose the path of democracy, or suffer the destruction of the Islamic government.
Correspondents say tension has been rising in Iran, with frustration at the slow pace of reforms promised by President Khatami.
Students last held mass protests at the end of last year in anger at a death sentence passed against reformist lecturer Hashem Aghajari for alleged blasphemy.
They were later backed by 120 Iranian MPs who criticised the punishments meted out to the student leaders.
Some observers also say the ruling clerics are trying to extend their power after the overthrow of Saddam Hussein in neighbouring Iraq.
Despite threats from the United States not to meddle in Iraq, religious leaders invited a militant Shia cleric Moqtada Sadr to Tehran for talks last week.
Mr Sadr has publicly backed the idea of an Iranian-style theocracy in Iraq, an idea denounced by the US.