Amateur video shows tear gas fired in Tehran protest
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.
US President Barack Obama urged Iran's government "to stop all violent and unjust actions against its own people", saying the "world is watching".
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.
Some reports could not be independently confirmed. Foreign news organisations - including the BBC - have been subjected to strict controls which prevent reporters from leaving their offices.
A BBC correspondent in Tehran says his impression is that the police have broken up large crowds into smaller groups to prevent them assembling.
Early on Saturday, the wife of Mr Mousavi and an aide to another rival candidate, Mehdi Karroubi, said the rally would go ahead, although this was later contradicted by his party.
Speaking on state TV, deputy police chief Ahmad Reza Radan warned police would "certainly fight against any form of illegal gathering and protest". He also said protest organisers would be arrested.
The result triggered almost daily street protests - a challenge to ruling authorities unprecedented since the Islamic revolution of 1979.
Mr Mousavi had been expected, along with fellow challengers Mr Karroubi and Mohsen Rezai, to discuss more than 600 objections they had filed complaining about the poll at a meeting of the Guardian Council, which certifies elections, on Saturday.
But neither Mr Mousavi nor Mr Karroubi attended the meeting - which suggests, our correspondent says, they have abandoned their legal challenge to the election results.
State TV quoted the Guardian Council as saying it was "ready" to recount a randomly selected 10% of ballot boxes.
It had previously offered a partial recount of disputed ballots from the election, rather than the full re-run of the election demanded by protesters.
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