Protestors rarely rise in these numbers against the Mubarak administration which has been known in the past for crackdowns, particularly on supporters of Islamic radicals as well as on blogger personalities known for scathing critiques of government policies. Although current reports describe a largely peaceful protest, there has been some push back from Egyptian security including tear gas. Many protestors got their cue from a protest planner’s Facebook page.
The situation is very much fluid at the moment. The Guardian is providing excellent updates on its blog. Videos abound.