The opposition called for a demonstration Monday in solidarity with Egypt's popular revolt that a few days earlier forced the president there to resign after nearly 30 years in office. The rally is the first major show of strength for Iran's cowed opposition in more than a year.
Police used tear gas against the protesters in central Tehran's Enghelab, or Revolution, square and in Imam Hossein square, as well as in other nearby main streets. Demonstrators responded by setting garbage bins on fire to protect themselves from the stinging white clouds.
Eyewitnesses said at least three protesters injured by bullets were taken to a hospital in central Tehran while dozens of others were hospitalized because of severe wounds as a result of being beaten.
"An Iranian dies but doesn't accept humiliation," demonstrators chanted. "Death to the dictator," they said, in a chant directed at hardline President Mahmoud Ahmadinejad.
Security forces on motorcycles could also be seen chasing protesters through the streets, according to eyewitnesses.
The semiofficial Fars news agency called protesters "hypocrites, monarchists, ruffians and seditionists," and ridiculed them for not chanting any slogans about Egypt as they had originally promised.
Fars, which is linked to the Revolutionary Guard, Iran's most powerful military force, said an unspecified number of protesters were arrested and handed over to police and security officials. It didn't elaborate.
Opposition website kaleme.com reported that similar rallies took place in the central city of Isfahan and Shiraz in the south. Security forces used force to disperse them as well.
Foreign media are banned from covering street protests in Iran.
Following the announcements by the opposition that they would attempt to hold a new rally in solidarity with the Egypt uprising, Iran's security forces cut phone lines and blockaded the home of an opposition leader in attempts to stop him attending the planned rally.
Police and militiamen poured onto the streets of Tehran to challenge the marches, which officials worry could turn into demonstrations against Iran's ruling system.
The security clampdown is reminiscent of the backlash that crushed a wave of massive protests after Ahmadinejad's disputed re-election in June 2009. But opposition supporters revived a tactic from the unrest, shouting "Allahu Akbar," or God is Great, from rooftops and balconies into the early hours Monday in a sign of defiance toward Iran's leadership.
The reformist website kaleme.com said police stationed several cars in front of the home of Mir Hossein Mousavi ahead of the demonstration called for Monday in central Tehran.
Mousavi and fellow opposition leader Mahdi Karroubi have been under house arrest since last week after they asked the government for permission to hold a rally on Feb. 14 in support of the uprisings in Egypt and Tunisia.
On Sunday, the opposition renewed its call to supporters to rally, and accused the government of hypocrisy by voicing support for the Egyptian and Tunisian uprisings while refusing to allow Iranian political activists to stage a peaceful demonstration.
Across central Tehran, riot police, many on motorbikes, fanned out to prevent any demonstration, witnesses said, speaking on condition of anonymity because of fears of reprisals from authorities.
The uprising in Egypt opened a rare chance for the political gambit by Iran's opposition.
Ahmadinejad claimed the Egyptians who toppled President Hosni Mubarak took inspiration from Iran's 1979 Islamic Revolution, which brought down a Western-backed monarchy. Iran's opposition movement used the comments to push the government into a corner and request permission to march in support of Egypt's protesters.
Iranian officials quickly backpedaled and said no pro-Egypt rallies were allowed - bringing sharp criticism from the White House and others.
Karroubi and Mousavi have compared the unrest in Egypt and Tunisia with their own postelection protest movement. Mousavi said Iran's demonstrations were the starting point but that all the uprisings aimed at ending the "oppression of the rulers."
Turkish President Abdullah Gul, who is on a visit to Iran, urged governments in the Middle East to listen to the demands of their people.
"When leaders and heads of countries do not pay attention to the demands of their nations, the people themselves take action to achieve their demands," IRNA quoted Gul as saying Monday.
Hundreds of thousands of Iranians peacefully took to the streets in support of Mousavi after the June 2009 vote, claiming Ahmadinejad was re-elected through massive vote fraud.
A heavy government crackdown suppressed the protests. The opposition has not been able to hold a major protest since December 2009.
The opposition says more than 80 demonstrators were killed in the turmoil. The government, which puts the number of confirmed deaths at 30, accuses opposition leaders of being "stooges of the West" and of seeking to topple the ruling system through street protests.
State TV said Mousavi and Karroubi were carrying out a plot designed by Iran's enemies.