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 have been planning for days through the Internet. Heavy police forces deployed at key points in the city 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 headscarves and young men dashed away, rubbing their eyes in pain as police fired tear gas, in footage aired on state-run Press TV. In a photo from Thursday's events in Tehran obtained by The Associated Press outside Iran, a woman with her black headscarf looped over her face thrust her fist into the air in front of a garbage bin that had been set on fire.
In another image, a man dropped to his knees, overcome by the effects of tear gas.
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" and waved their hands in the air, witnesses said. The crowd grew to nearly 1,000 people, the witnesses said.
"Police, protect us," some of the demonstrators chanted, asking the forces not to move against them.
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 passers-by. 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 past 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, passers-by 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. Iranian authorities have imposed restrictions that ban reporters from leaving their offices to cover demonstrations.
Many of the marchers were young men and women, some wearing green surgical masks, the color of Mousavi's movement, but older people joined them in some places. Vehicles caught in traffic honked their horns in support of the marchers, witnesses said. Police were seen with a pile of license plates, apparently pried off honking cars in order to investigate the drivers later, the witnesses said.
Soon after the confrontations began, mobile phone service was cut off in central Tehran, a step that was also taken during the height of the postelection protests to cut off communications. Mobile phone messaging has been off for the past three days, apparently to disrupt attempts at planning.
The calls for a new march have been circulating for days on social networking Web sites and pro-opposition Web sites. Opposition supporters planned the marches to coincide with the anniversary Thursday of a 1999 attack by Basij on a Tehran University dorm to stop protests in which one student was killed.
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 that has continued even since the previous crackdown.
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 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 1,000 people were arrested in the crackdown and 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 remainder have been released, Najafabadi said.
Arrests have continued over the past week, with police rounding up dozens of activists, journalists and bloggers.
In the latest detentions, prominent human rights lawyer Mohammad Ali Dadkhah was taken away by security forces from his office Wednesday along with his daughter and three other members of his staff, the pro-opposition news Web site Norouz reported. A former deputy commerce minister in a previous pro-reform government, Feizollah Arab-Sorkhi, was also arrested at his Tehran home, the site reported.
A large number of top figures in Iran's reform movement, including a former vice president and former Cabinet members, have been held for weeks since the election.
Iranian authorities have depicted the postelection turmoil as instigated by enemy nations aiming to thwart Ahmadinejad's re-election, and officials say some of those detained confessed to fomenting the unrest. Opposition supporters say the confessions were forced under duress.
Ahead of the protests, Tehran's governor Morteza Tamaddon accused "foreign counterrevolutionary networks" of plotting new marches. "If some individuals plan to carry out any anti-security actions by listening to (protest) calls ... they will be smashed under the feet of our aware people," he said late Wednesday, according to the state news agency IRNA.