Some Basij militiamen, who are being used as street-level enforcers, stood in groups armed with clubs while others rode around on motorbikes.
BBC Persian TV received an e-mail from one person saying: "There are lots of people but they are scattered, and lots of police guards. They are firing bullets in the air and using tear gas against the crowds."
In side streets, young people set fire to rubbish skips in what they said was a protest but also an attempt to combat the tear gas.
Some claimed not to be scared by the large show of force and threats from the Revolutionary Guards.
The Guards have close ties to the country's supreme leader.
Video has emerged of Iranian police making arrests on Saturday
In a statement posted on their website, they said their troops would break up street protests and force protesters from the streets.
"Be prepared for a resolution and revolutionary confrontation with the Guards, Basij and other security forces and disciplinary forces," they said.
"The Guards will firmly confront in a revolutionary way rioters and those who violate the law," they added.
The Basij militia was involved in quelling earlier protests during more than a week of demonstrations against the re-election of President Mahmoud Ahmadinejad.
In other developments on Monday:
• Italy instructed its embassy in Tehran to provide humanitarian aid to protesters wounded during clashes
• The UK Foreign Office said it was evacuating the families of staff based in Iran "until the situation improves"
'No memorial service'
The fiance of Neda Agha-Soltan, the young woman whose violent death during clashes in Tehran on Saturday was recorded on video and uploaded to the internet, has described the events leading up to her shooting in an interview for BBC Persian TV.
The authorities are aware that everybody in Iran and throughout the whole world knows about her story
She had been sitting with her music teacher in a car, stuck in traffic, when she decided to get out because of the heat.
"She got out of the car for just for a few minutes [and] that's when she was shot dead," said Caspian Makan.
Mr Makan quoted eyewitnesses as saying she appeared to have been targeted deliberately by "paramilitaries in civilian clothing".
He added that officials had prevented mourners holding a memorial service at a mosque on Monday.
"The authorities are aware that everybody in Iran and throughout the whole world knows about her story," he told the BBC. "They were afraid that lots of people could turn up."
Election results show Mr Ahmadinejad won the 12 June election by a landslide, taking 63% of the vote, almost double that of Mir Hossein Mousavi, his nearest rival.
Following complaints, the powerful Guardian Council, which oversees the electoral process, said it had found some evidence of voting irregularities but the number had "no effect on the result of the elections", it insisted.
An independent British analysis of the disputed election results has found irregularities in the reported turnout, as well as "implausible" swings in the vote in favour of Mr Ahmadinejad.
Analysts from St Andrew's University and the Chatham House think-tank said votes in favour of Mr Ahmadinejad in a third of the provinces would have required an "unlikely scenario" of voting patterns.
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