Iranian police have used water cannon, batons, tear gas and live rounds to break up protests over the presidential election, witnesses in Tehran say.
A BBC reporter said he saw one man shot and others injured amid running fights.
Defeated candidate Mir Hossein Mousavi repeated calls for the election to be annulled on the grounds it was rigged.
Security forces were everywhere in central Tehran in the late afternoon and early evening.
As I spent a couple of hours driving around in heavy traffic I could see thousands of men, some uniformed members of the military riot squads, some units of revolutionary guard, and everywhere basijis - militiamen who look like street toughs.
The security men were deployed on every street corner, in long lines down the sides of the roads, and in all the main squares.
The basijis wore riot helmets and carried big clubs. It was designed to intimidate, and while I was there, it was working.
BBC eyewitness: Heavy security
The country's supreme leader Ayatollah Khamenei had warned protesters on Friday not to continue their rallies, but they openly defied his words.
It was unclear if Iranian political leaders had backed their supporters continuing to march.
In a letter to the electoral body, the Guardian Council, Mr Mousavi, who had not made a public comment for two days, reiterated his calls for the election to be declared void.
He alleged the vote, held on 12 June, was rigged months previously.
Official results of the presidential poll gave President Mahmoud Ahmadinejad a resounding 63% of votes, compared to 34% for Mr Mousavi, his nearest rival.
In other developments:
Thousands of police, militia and secret policemen blocked access to Enghelab and Azadi squares, and protesters were throwing stones in surrounding streets
A BBC correspondent saw one man shot in a crowd and another with injuries from a razor-wielding secret policeman
About 3,000 protesters were reportedly gathered at Enghelab Square, according to Associated Press news agency. They chanted "Death to the dictator" and "Death to dictatorship"
One witness told Reuters news agency that protesters loyal to defeated candidate Mir Hossein Mousavi set fire to a building in southern Tehran used by backers of President Mahmoud Ahmadinejad
A column of black smoke is hanging over the city centre, our correspondent says
Two Iranian news agencies reported that the suicide bomber died and two people were injured in the bombing at the shrine of Ayatollah Khomeini, leader of the 1979 revolution.