CAIRO — Security forces fired tear gas and clashed today with several thousand protesters in Cairo's Tahrir Square in the third consecutive day of violence that has killed at least 24 people and has turned into the most sustained challenge yet to the rule of Egypt's military.

Throughout the day, young activists demanding the military hand over power to a civilian government skirmished with black-clad police, hurling stones and firebombs and throwing back the tear gas canisters being fired by police into the square, which was the epicenter of the protest movement that ousted President Hosni Mubarak.

The night before saw an escalation of the fighting as police launched a heavy assault that tried and failed to clear protesters from the square.

In a show of the ferocity of the assault, the death toll quadrupled from Sunday evening until this morning. A constant stream of injured protesters were brought into makeshift clinics set out on sidewalks around the square.