This began as a peaceful protest by Copts in reaction to some earlier incidents as well as the military government. Some reports say that at some point, the protesters were attacked by "thugs" throwing rocks, etc. At that point Egyptian security forces stepped in. Vehicles were set on fire, people were run over, a soldier was killed and the security forces began firing on the crowd. Details are still sketchy.
The death toll varies from 6 to 17, with possibly dozens injured. Attacks on Copts aren't uncommon but this is a much larger incident than usual.