Hundreds of people armed with knives and machetes have attacked pro-democracy activists in Cairo's Tahrir Square, Egypt's state TV reports.
The footage showed stones being thrown from both sides at the square. At least two people have been injured.
The people who entered the square wanted to force the activists continuing a sit-in out of the square, reports say.
Tahrir Square was the epicentre of protests that forced President Hosni Mubarak to step down last month.
Activists have continued to gather in the square even after Mr Mubarak's departure. They are demanding a complete break with the Mubarak regime.