Tehran, Iran ? Thousands of protesters streamed down avenues of the capital Thursday, chanting “death to the dictator” and defying security forces who fired tear gas and charged with batons, witnesses said.

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.