TEHRAN, Iran -- Violent clashes erupted yesterday in downtown Tehran between thousands of defiant protesters chanting "Death to the dictator" and security forces wielding truncheons, as the political crisis over Iran's disputed presidential election stretched into its fourth week.

Large contingents of uniformed and plainclothes security forces flooded the city's central squares and managed with batons and tear gas to eventually disperse demonstrators, many of whom wore black and held up their fingers in "V" for victory salutes.

Protesters in and around Enqelab (Revolution) Square set fire to trash bins to ward off the effects of gas, witnesses said, but by and large avoided engaging the security forces in the rock-throwing and running street battles that characterized protests several weeks ago.

There were reports of clashes in other Iranian cities, but they could not be confirmed.

Demonstrators said they were determined to defy authorities, led by Supreme Leader Ayatollah Ali Khamenei, who have demanded an end to public rallies and acts of civil disobedience. Tracts distributed online and handed out as leaflets called on protesters to borrow nonviolent tactics used in the U.S. civil rights movement, avoid internal rifts in their ranks and take no action that would hamper morale.

Protesters chanted in support of Mir Hossein Mousavi, who was defeated by President Mahmoud Ahmadinejad in a disputed election last month, and urged the security forces to join them.

"Mojtaba, may you die, so that you don't get the supreme leadership," went one chant, referring to Ayatollah Khamenei's son, who is said to be behind the crackdown on dissent and angling for his father's position.

Though the numbers of protesters did not nearly match the hundreds of thousands who flooded streets for peaceful demonstrations following the June 12 election, organizers showed yesterday that they could quickly gather a crowd despite the efforts of security forces.

The day, the 10-year anniversary of a student uprising violently crushed by the government, is a perennial occasion for confrontations between demonstrators and police.

"It is going to continue," vowed one marcher in his 60s, big drops of sweat on his forehead. "They have killed our dear youth. How we can forgive them?"

Many of the demonstrators wore surgical masks to protect their identities from cameras stationed at adjacent buildings. They could be seen escaping into side streets and regrouping as shops quickly were shuttered. Some witnesses said pro-government Basiji militiamen could also be seen wearing masks to hide their faces from protesters' cameras.

Passing drivers and motorcyclists honked their horns and flashed the victory sign in support of the clumps of demonstrators.

Uniformed security forces wearing black helmets and riding motorcycles as well as plainclothes officers had blocked off streets around Enqelab Square, near the Tehran University epicenter of the protest. One witness described shots fired in the air near Vali Asr Square, another downtown crossroads where demonstrators had planned to gather in case Enqelab was inaccessible.

But as the militiamen tried to drag away demonstrators, one witness said protesters would join together to overpower them and rescue comrades. He also said he saw some women with their head scarves pulled off being forced into police vans. Another woman taking pictures with her cell phone camera was dragged away.

One witness described how five Basiji militiamen pummeled an elderly lady who loudly warned them that they would receive their comeuppance on Judgment Day.

Police managed to quell demonstrators within three hours, witnesses said.

"We will wait to see the reflection of today's event in the local and international media and the responses of the authorities," said Farzad, 28, who asked that his last name not be used. "Then we get together with our fellow demonstrators and decide another contingency plan for further struggles."