For days, supporters of opposition leader Mir Hossein Mousavi have been calling for new protests in Tehran and other cities on Thursday, their first significant attempt to get back on the streets since security forces crushed massive demonstrations nearly two weeks ago in Iran's postelection turmoil.
Tehran governor Morteza Tamaddon warned that any new march Thursday would meet the same fate.
"If some individuals plan to carry out any anti-security actions by listening to calls by counterrevolutionary networks, they will be smashed under the feet of our aware people," he said, according to the state news agency IRNA in a report late Wednesday.
Thursday afternoon, a stepped-up number of uniformed policemen along with plainclothes Basiji militiamen stood at intersections all along Revolution Street and at nearby near Tehran University, some of the sites where protests were called.
Still, a group of around 300 young people gathered in front of Tehran University and began to chant, "Death to the dictator," witnesses said. Many of them wore green surgical masks, the color of Mousavi's movement.
Police charged at them, swinging batons, but the protesters fled, then regrouped at another corner and resumed chanting, the witnesses said. Police chased them repeatedly as the protesters continued to regroup, the witnesses said, speaking on condition of anonymity because they feared government retribution.
Within an hour, the number of protesters grew to about 700 and marched toward the gates of Tehran University, the witnesses said. A line of policemen blocked their path, but they did nothing to disperse the gathering as the protesters stood and continued to chant, the witnesses said.
At another location, on Valiasr Street, around 200 protesters gathered, and police fired tear gas to disperse them, but the demonstrators sought to regroup elsewhere, the witnesses said.
Soon after the confrontations began, mobile phone service was cut off in Tehran, a step that was also taken during the height of the post-election protests to cut off communications. Mobile phone messaging has been cut in the country for the past three days.
They were the first such protests in 11 days, since the crackdown though 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.
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.
Ahead of Thursday's planned march, authorities appeared to have taken a number of other steps to prevent participation, including the halting of SMS messaging. The government also closed down universities and called a government holiday on Tuesday and Wednesday, citing a heavy dust and pollution cloud that has blanketed Tehran and other parts of the country this week.
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.
Supreme leader Ayatollah Ali Khamenei declared the results valid after a partial recount and warned that unrest would not be tolerated.
In the crackdown since the election, at least 20 protesters and 7 Basijis were killed.
Police have said 1,000 people were arrested and that most have since been released. But the state-run English language news network Press TV quoted prosecutor-general Qorban-Ali Dorri Najafabadi saying Wednesday that 2,500 people were arrested and that 500 of them could face trial. The remainder, he said, have been released.
Arrests have continued over the past week, with police rounding up dozens of activists, journalists and bloggers.
In the latest detentions, a 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.