TEHERAN - IRANIAN police fired tear-gas on Thursday as thousands of demonstrators defied government warnings and staged a march to commemorate the anniversary of bloody student unrest in 1999, witnesses said.

The witnesses said police arrested several people, and some protesters set roadside rubbish bins ablaze.

They said windows in a state-owned bank were smashed, and police seized the number plates of vehicles whose drivers sounded their horns in protest.

Reinforcements were sent in after a volley of tear-gas failed to disperse the demonstrators who continued to grow in number, the witnesses said. Police then fired a second volley.

The witnesses said members of the hardline Basij militia also reinforced police ranks.

Officers in riot gear deployed in force to try to quell any gathering as tensions remained high following the wave of protests over June's disputed re-election of hardline President Mahmoud Ahmadinejad that saw hundreds of thousands take to the streets.

Some of the protestors chanted slogans in support of Ahmadinejad's defeated challenger, Mir Hossein Mousavi, who has denounced last month's election as a 'shameful fraud'.

The authorities had warned of a harsh response to any commemoration of the 1999 violence in which at least one student was killed when hardline vigilantes stormed student dormitories, according to an official toll.

The warning came after the G-8 world powers expressed 'serious concern' over last month's post-election violence which left at least 20 people dead.

Groups of students have held small commemorative gatherings in previous years, but Teheran governor Morteza Tamadon issued a blunt warning this year.

'If some people make moves that are contrary to security initiatives under the influence of anti-revolutionary networks, they will be trampled under the feet of our alert people,' he told the official IRNA news agency. -- AFP