Egyptian troops blasted protesters with water cannons, tear gas and live ammunition, trying to prevent them from marching on the Defence Ministry in clashes that left one soldier dead and scores of people injured just three weeks ahead of presidential elections.
The fierce street battles raised fears of a new cycle of violence surrounding the upcoming vote to replace Hosni Mubarak, who was ousted more than a year ago.
For the first time in Egypt's chaotic transition, hard-line Islamists, rather than secular forces, were at the forefront of the confrontation with the military rulers who have been accused of trying to cling to power.
The military council imposed an 11pm to 7am curfew on the area surrounding the Defence Ministry, which has emerged as a flashpoint for the protesters' anger after nine people were killed in clashes between unidentified assailants and protesters who mainly comprised supporters of a disqualified Islamist presidential candidate.
The violence has thrown the campaign for the May 23-24 elections into turmoil, with two front-runners and several other candidates temporarily suspending their campaigns to protest the military's handling of the situation.
Thousands of demonstrators massed in Cairo's Tahrir Square - the epicentre of last year's popular uprising - earlier for what has become a weekly rally to demand that the generals speed up a transition to civilian rule.
Protesters included the powerful Muslim Brotherhood and ultraconservative Islamists known as Salafis but also revolutionary youth who spearheaded the mass rallies that ousted Mr Mubarak.