Mubarak isn't stupid, he can think as well as any news correspondent. One of the things people have commented about has been the difference between Tunisia and Iran's "Green Revolution". The main difference people could come up with was that the Iranian regime really did have a large base. Not a majority perhaps, but enough to put their own crowds on the street and to provide them with muscle.
Well, Mubarak's got his crowd on the street now. My guess is that it's not so much a "crowd", but a collection of police and security officers, the core force that Mubarak has used to suppress dissent. al-Jazeera has several cameras on the action, which is mostly limited at this point to the hurling of rocks and other impromptu projectiles, but appears to be escalating with the fall of night.
The pro-Mubarak side came prepared and armed for bear. They look like they brought with them at least three two-and-a-half ton 6x6 trucks, possibly from army stock, which they are using as a rolling defensive line, giving them a moving barricade to shield their lead ranks from rocks, and some additional horsepower to run down barricades. It looks like they've taken the 6th October Bridge, and are trying to push their way into Tahrir Square from the north, going down the Meret Basha. It's a major approach, and the western side of the road is anchored on the Egyptian Museum, currently occupied by the Egyptian Army, so neither side can go around the flank. They've also taken the intelligent step of seizing the rooftops along the eastern edge of the street, giving them a platform to rain projectiles down on the anti-government protesters who still hold the square.
Meanwhile, the army seems disinclined to get in the middle of this. To me it looks like they're waiting to see what happens. And it looks like the Molotov Cocktails are starting.
( Cut For Amateur Musings Because I'm Sure Nobody Else Cares )