CAIRO — As least 11 people were killed after assailants attacked protesters staging a sit-in near Egypt’s Ministry of Defense early on Wednesday, setting off hours of clashes that threw the coming presidential election into disarray as at least five candidates announced the suspension of their campaigns to protest the deaths.

The victims, killed by gunfire, clubs or knives, included a third- year medical student from Luxor and several young men from the Cairo neighborhood Abbaseya, where the fighting occurred, doctors said. It went for hours with no intervention from the authorities — suggesting to some at least a degree of government complicity — as opposing sides fought a pitched battle hurling stones and incendiary devices, turning a residential neighborhood into a war zone marked by scattered fires.

At around 1 p.m., the security services suddenly arrived and the fighting stopped.

The confusing, lethal episode has widened a rift between Egypt’s military rulers and protesters who are pushing for a speedy transition to a civilian government, with the protesters convinced that their assailants were in the employ — or at least doing the bidding — of the military. And the event cast a shadow over Egypt’s coming presidential election, shortening an already brief campaign season as it sharpened the differences between candidates.

One of the presidential front-runners, Abdel Moneim Aboul Fotouh, a former Muslim Brotherhood leader who has attracted leftists and liberals with his candidacy, suspended his campaign indefinitely, writing on Twitter, “We can’t discuss tomorrow while our youth are drowning in their blood today.”

One of his main rivals, Amr Moussa, a former Egyptian minister, announced that he was not suspending his campaign but that he was stopping television advertising temporarily and had “canceled many events.” In a statement, Mr. Moussa criticized the security services for standing “idly by.”

The two men were scheduled to appear in a much-anticipated debate on Thursday night, but that too was delayed because of the violence. Yosri Fouda, a journalist who was to moderate the debate, announced on his Twitter account that it would be postponed until next week.

Four other candidates also suspended their campaigns, including Mohamed Morsi, the Muslim Brotherhood’s candidate; Khalid Ali, a human rights lawyer; and Hamdeen Sabbahi, an activist and founder of a Nasserist party.

In a morning briefing for foreign reporters, Mr. Morsi warned the ruling military council against using the violence as an excuse to delay the elections, reflecting a widespread fear that the council is looking for a pretense to retain power. But his comments also reflected an intensifying power struggle between the Muslim Brotherhood and the military as the presidential vote approaches.

“We hold the military council responsible first, because it’s the one running the country,” Mr. Morsi said. “It’s the one with the authority in its hand. We will not allow for the presidential elections to be postponed at all.”

In statements released through official outlets, the military did not exactly assuage the fears. A member of Parliament, Mustapha Bakri, quoted the army chief of staff, Sami Anan, as telling Mr. Bakri that the military was “considering” handing over power on May 24, if the first round of voting yielded an outright winner.

In the statements, the ruling council also promised not to harm protesters as it deployed its troops. The statement did not address why the security services took hours — perhaps as many as 12 — before responding to the violence.

The clashes followed days of a simmering standoff near the Ministry of Defense, in what began as a sit-in by supporters of a disqualified presidential candidate, Hazem Salah Abu Ismail, an ultraconservative Islamist known as a Salafi. That sit-in grew into a broader protest against the ruling military council which was joined by revolutionary youth groups. At least one person was killed during the sporadic clashes over the last few days.

No one could say for sure who the assailants were. Many protesters were adamant that their attackers were affiliated with some branch of the security services, though they had little hard evidence. “The thugs were shooting tear gas at us,” said Sherif Mohammed, 26, a member of the April 6 youth movement. “It’s not reasonable that civilians would have tear gas guns.”

Several people said the clashes on Wednesday began at about 1 a.m. “We found thugs attacking us suddenly and throwing rocks,” said Mohammed Rifiqi, 21, another April 6 member. He said security officers who had been standing watch at a nearby mosque disappeared. The assailants fired tear gas and live ammunition, according to Mr. Rifiqi.

By 11 a.m., a residential intersection had become the front line of a fierce battle, with the protesters throwing rocks gathered by volunteers and hurling incendiary devices with slingshots. Periodically, there was a stampede, as bursts of birdshot came from the other side. Doctors gave death tolls ranging from 11 to 13. Dr. Saleh Mohamed, who worked at a field hospital near the clashes, said he treated five patients who died of gunshot wounds to the head. Several doctors spoke of at least one man whose throat was cut. Of the nearly 200 injuries reported by officials, several people were blinded by birdshot, Dr. Mohammed said.

As the protesters started pushing their attackers back, they captured a man. “He’s a thug, from over there,” one protester said. A few people tried to take the man away, if only to stop him from being beaten by the crowd, as people hit him, with sticks, empty fruit boxes and plastic helmets and blood poured from his head.

Hours after the fighting stopped, hundreds of people marched to the scene of the clashes, chanting slogans against military rule. They included Ahmed Obyda, a 20-year-old student, who carried a sign with a line by the Egyptian poet, Abdul Rahman el-Abnoudy.

“Kill me,” it said. “My killing will not bring back your state.”

Liam Stack contributed reporting. (Copyright 2012 The New York Times Company 05/03/12)