Tehran is bracing for another day of tension after Iran's capital descended into the worst violence since the country's disputed presidential election.

After warnings against protesting from Iran's Supreme Leader, security forces have been using increased force to crack down on the latest protests.

There is no sign of a political resolution to the opposition's complaints of vote-rigging in favour of President Mahmoud Ahmadinejad and demonstrations have been continuing.

Helicopters criss-crossed the city and ambulance sirens wailed into the night after streets emptied of protesters who had defied Friday's stern warning from Supreme Leader Ayatollah Ali Khamenei against further demonstrations.

Riot police had deployed in force, firing teargas, using batons and water cannon to disperse groups of several hundred Iranians who had gathered across the city.

Foreign news organisations are banned by the authorities from going to unauthorised demonstrations but media outlets have been receiving reports from protesters themselves.

This man, who calls himself Siavoush, has been speaking to the BBC's Persian service.

"We witnessed security forces and I couldn't tell if they were in uniform or plain clothing opening fire directly on the crowd," he said.

"All we could do was run away and I saw one person wounded on the ground.

"We kept hearing different things from those around us about the number of people killed or injured."

'Be peaceful'

The main opposition challenger in the disputed Iranian election Mir-Hossein Mousavi has urged his supporters to continue their protests, but stressed that they should be peaceful.

In a message on his website, Mr Mousavi says he does not want anyone to put their lives in danger, but says they have a constitutional right to demonstrate.

Mr Mousavi is urging Iranians to stage a national strike if he is arrested by the authorities.

Mr Mousavi says the June 12 elections that delivered an overwhelming victory to hardline anti-Western President Ahmadinejad were fraudulent and must be annulled.

He says the fraud was months in the planning.

Obama

US President Barack Obama called on Iran's Government to stop "violent and unjust" action against its own people, in his sharpest challenge yet to top leader on a day of bloodshed and violence.

Mr Obama issued a written statement stiffening his position on the turmoil, after a week seeking to find the right tone to recognise unprecedented protests in Iran while avoiding being seen to meddle in Iranian affairs.

"We call on the Iranian Government to stop all violent and unjust actions against its own people," Mr Obama said, as protestors clashed in defiance of an order from supreme leader Ayatollah Ali Khamenei for an end to protests.

"The Iranian Government must understand that the world is watching. We mourn each and every innocent life that is lost," Mr Obama said.

Worldwide support

Iranians around the world are holding rallies to show their support for the demonstrators in Tehran.

Hundreds of Iranian expats are protesting outside the Iranian embassy in London to demand their absentee ballots are counted.

In Australia, hundreds of protesters gathered in Sydney and Brisbane, calling on the Government not to recognise the re-election of Mr Ahmadinejad.

This Iranian-American man has been protesting in New York every day since the election results were announced.

"This is a movement which is of course generated on the foundation of the resentment of the Iranian people, especially the younger generation that demands freedom," he said.

- ABC/BBC/Reuters