Turning garbage bins into burning barricades and darting through choking clouds of tear gas, the opposition made its first foray into the streets in nearly two weeks in an attempt to revive mass demonstrations that were crushed in Iran's postelection turmoil.

Iranian authorities had promised tough action to prevent the marches, which supporters of opposition leader Mir Hossein Mousavi had been planning for days through the Internet. Heavy police forces deployed at key points ahead of the marches, and Tehran's governor vowed to "smash" anyone who heeded the demonstration calls.

In some places, police struck hard. Security forces chased after protesters, beating them with clubs on Valiasr Street, Tehran's biggest north-south avenue, witnesses said. Women in head scarves and young men dashed away, rubbing their eyes in pain as police fired tear gas, in footage aired on state-run Press TV.

But the clampdown was not total. At Tehran University, a line of police blocked a crowd from reaching the gates of the campus but then did not move to disperse them as the protesters chanted "Mir Hossein" and "Death to the dictator," witnesses said. The crowd grew to nearly 1,000 people, the witnesses said.

The protesters appeared to reach several thousand, but their full numbers were difficult to determine, since marches took place in several parts of the city at once and mingled with passersby. There was no immediate word on arrests or injuries.

It did not compare to the hundreds of thousands who joined the marches that erupted after the June 12 presidential election, protesting what the opposition said were fraudulent results. But it was a show of determination despite a crackdown that has cowed protesters, who have not held a significant rally for the last 11 days.

Onlookers and pedestrians often gave their support. In side streets near the university, police were chasing young activists, and when they caught one, passersby chanted "Let him go, let him go," until the policemen released him. Elsewhere, residents let fleeing demonstrators slip into their homes to elude police, witnesses said.

All witnesses spoke on condition of anonymity for fear of government reprisals.

Demonstrators dispersed by nightfall. But after sunset, shouts of "Death to the dictator" could be heard from rooftops around the city, a half-hour nightly ritual by Mousavi supporters.

Mousavi and his pro-reform supporters say he won the election, which official results showed as a landslide victory for incumbent hard-liner Mahmoud Ahmadinejad. Days of massive demonstrations erupted, until Iran's supreme leader, Ayatollah Ali Khamenei, declared the results valid and warned that unrest would not be tolerated.

In the crackdown that followed, at least 20 protesters and seven Basijis were killed, according to police. Police have said that 1,000 people were arrested but that most have since been released. But prosecutor-general Qorban-Ali Dorri Najafabadi said Wednesday that 2,500 people were arrested and that 500 of them could face trial, Press TV reported. The rest have been released, Najafabadi said.