There have been reports of some shootings in Iran. The government denies it.
The important part is that there seems to be some level of acceptance in the protest movement that blood will be spilled, and a recognition that unlimited violence by the regime could backfire:
The security forces clearly have to tread a fine line between not appearing weak but also not provoking opposition protesters, says Siavash Ardalan of BBC Persian TV.
Police helicopters were seen flying over central Tehran as clouds of black smoke billowed into the sky, reports said.
On the ground, the security forces clashed with protesters trying to reach central Enghelab Square, witnesses said.
Protesters were chanting, "This is the month of blood", and calling for the downfall of Iran's Supreme Leader Ayatollah Ali Khamenei, according to opposition websites.
At the same time, crowds of pro-government demonstrators marched on Enghelab Street to voice support for Ayatollah Khamenei, witnesses said.
So what's next?