Yesterday, at about 2pm, as I was leaving Midan Tahrir, we started seeing groups of what the media is calling, for lack of a better word, "pro-Mubarak supporters" arriving along Kasr El Aini Street. They were also coming from other directions, all converging on the square. In the morning I'd seen arguments breaking out in the square between some people who had come to defend Mubarak and the protesters--they were heated, but entirely peaceful.
The crowds that arrived in the afternoon, as I think everyone has seen on TV by now, were anything but--they attacked the protesters, charging them on horseback (!), throwing molotov cocktails at them and climbing to the tops of surrounding buildings to drop stones on them. The Wall Street Journal has a long, detailed account of the day's clashes.
I've been working in Egypt for 7 years and I know that "pro-Mubarak supporters" are always trouble. Everytime there is an election or a demonstration, somewhere in the vicinity, alongside the police, you'll see rows of shabbily dressed young men, waiting to be given the order to rough people up -- what people here simply refer to as "beltagiya" (thugs"). They are made up of petty criminals; police informants; unemployed youth; people over which the regime has some leverage. In 2004 I spoke to a young man who told me his boss (an NDP businessman) had simply told all his employees to go out and demonstrate for Mubarak or they would be fired. We often hear reports of these guys being offered little incentives--small amounts of cash, a chicken lunch
http://www.arabist.net/blog/2011/2/3/midan-tahrir-under-siege.html