From CNN, via The Guardian:
"I was going towards Baharestan with my friend. This was everyone, not just supporters of one candidate or another. All of my friends, they were going to Baharestan to express our opposition to these killings and demanding freedom. The black-clad police stopped everyone. They emptied the buses that were taking people there and let the private cars go on. We went on until Ferdowsi then all of a sudden some 500 people with clubs came out of [undecipherable] mosque and they started beating everyone. They tried to beat everyone on [undecipherable] bridge and throwing them off of the bridge. And everyone also on the sidewalks. They beat a woman so savagely that she was drenched in blood and her husband, he fainted. They were beating people like hell. It was a massacre. They were trying to beat people so they would die. they were cursing and saying very bad words to everyone. This was exactly a massacre... I don't know how to describe it."
More here, updated continuously.