"They were waiting for us," a source told CNN. "They all have guns and riot uniforms. It was like a mouse trap."
Witnesses reached outside the national Parliament building told The New York Times the confrontation was bloody and police used live ammunition. The protesters had defied government warnings and hundreds, perhaps thousands, descended on the square in front of parliament, the Times said.
A source told CNN about "500 thugs" with clubs came out of a mosque and attacked people, and the security forces were "beating women madly" and "killing people like hell."
Iran's Supreme leader, Ayatollah Ali Khamenei, insisted authorities would not yield to pressure from opponents demanding a new election following allegations of electoral fraud, the Times reported.
The Iranian government stepped up its claims that foreign governments were behind Iran's recent turmoil, saying several foreign nationals were arrested.
Iran's government-funded Press TV quoted the Intelligence Minister Gholamhossein Mohseni-Ezhei as saying some people or organizations with links to the West and Israel planned a series of bombings in Iran ahead of the June 12 election, CNN said.
President Mahmoud Ahmadinejad was declared the winner by a landslide in what challengers said was a rigged election. The results prompted massive demonstrations in Tehran and other Iranian cities.