The Mubarak supporters wield machetes, clubs, daggers and other weapons to attack anti-government protesters. According to the Guardian:
Both groups are pelting each other with rocks, it's extremely violent here...people are unsure about the army position...I don't see this coming to an end, it's been going on for hours now. There are hundreds of people injured, literally hundreds of people.
Nicholass Kristoff of the New York Times confirmed rumors that the pro-Mubarak thugs show signs of premeditation:
Today President Mubarak seems to have decided to crack down on the democracy movement, using not police or army troops but rather mobs of hoodlums and thugs. I’ve been spending hours on Tahrir today, and it is absurd to think of this as simply “clashes” between two rival groups. The pro-democracy protesters are unarmed and have been peaceful at every step. But the pro-Mubarak thugs are arriving in buses and are armed — and they’re using their weapons.