CAIRO - Egyptian troops blasted protesters with water cannons, tear gas, and live ammunition Friday, trying to prevent them from marching on the Defense 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 11 p.m. to 7 a.m. curfew on the area surrounding the Defense Ministry, which has emerged as a flashpoint for the protesters' anger after nine people were killed on Wednesday in clashes between unidentified assailants and protesters who mainly were 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 epicenter of last year's popular uprising - earlier Friday 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 Mubarak.

Despite official warnings against gathering, groups marched to the district of Abbasiyah to join a sit-in outside the Defense Ministry initially held by supporters of Hazem Abu Ismail. A lawyer-turned-preacher, hard-line Abu-Ismail was disqualified from the race because his late mother allegedly held dual Egyptian and American citizenship, making him ineligible under election laws. He has encouraged his followers to take to the streets. "We are in the face of a plot to abort the revolution," his spokesman Gamal Saber told the Al-Jazeera network on Friday.

The Health Ministry said one soldier was killed and 296 were injured. More than 170 people were arrested by the military, according to a security official, who spoke on condition of anonymity because he wasn't authorized to release the information.