The pictures from Tahrir Square tell the story.
This is a screenshot from Al Jazeera via Livestation at approximately 5:05 p.m. local Cairo time; the quality is grainy, but you can make out two trucks which have been moved into the street/square and individuals at close range heaving stones pulled from the pavement at each other.
The next photo from Al Jazeera’s website appears to have been taken before the trucks were moved into place; the anti-Mubarak and pro-Mubarak factions are more widely spaced apart.. . .
In the following photo by Evan C. Hill, believed to be an Al Jazeera reporter in Tahrir Square, shows the camels being used by pro-Mubarak forces to disrupt the peaceful protests.
Al Jazeera reports that sources say individuals have been paid the equivalent of US$17 each to join pro-Mubarak efforts. The following photo shows what are believed to be pro-Mubarak factions throwing projectiles from roofs onto anti-Mubarak protesters in Tahrir Square below.
A reporter on site for Al Jazeera-English says they cannot see any fires, however a video on Al Jazeera shows a storefront on fire. Other reports indicate the Egyptian museum may now be on fire as well, attributed to pro-Mubarak forces. Shots have been fired sporadically, believed to be warning shots attributed to the military.