December 27, 2009 was Ashura, traditionally a day of mourning for Shia Muslims, and in several Iranian cities nonviolent protests turned into clashes between anti-riot forces and opposition demonstrators. Among many others, the nephew of opposition leader Mir-Hossein Mousavi was killed.
There are plenty of other sources for further reading, but one of the most diligent English language live-bloggers during the recent violence was Andrew Sullivan, who writes for The Atlantic. Here are some scenes.
BE WARNED: SOME OF THESE VIDEOS ARE VERY DISTURBING.