[SeekAndFind]

Watch footage of the protest here ( there’s a video of Olsen collapsing):

http://www.youtube.com/watch?v=OZLyUK0t0vQ

[Nifster]

All I saw was tear gas being fired above the crowd. Then a pan to someone lying on the ground with no tear gas around him and then lots of confusion....he was hit in the head by something and someone. I am not about to suggest this was done by the police

[justlurking]

The view of Olsen collapsing is obscured, so it's hard to tell what caused it. Maybe someone else can advance it frame by frame and see something. But, the initial flashes (tear gas?) occurred 20 feet or more away. If it was one of those, it would have bounced off his head first.

However, you can clearly see police lob another grenade into the middle of the group of people that go back to Olsen's aid.

[Steve Van Doorn]

Nasty... note he may have been hit in the left eye.
in this video the opening shot at 0:01 you can see him walking

http://www.youtube.com/watch?v=QqNOPZLw03Q&feature=related

[Amberdawn]

Yes, at the 19-21 second mark, you can see him fall HARD.