(CNN) -- Assailants targeted protesters demonstrating against the exclusion of a presidential candidate in Cairo early Wednesday, killing at least 11, medical sources told CNN. At least 100 people were injured, said Hisham Shiha, the deputy minister of health. It was unclear who the attackers were, but they were not wearing uniforms, witnesses said.
Clashes were ongoing Wednesday afternoon. There were fears of increased violence as additional protesters marched from downtown to join the other demonstrators. The military dispatched a unit to attempt to restore calm. Protesters hurled rocks at the assailants, who responded with a steady barrage of rocks and Molotov cocktails.
"Down with military rule!" the protesters chanted.
Protesters camped outside the Ministry of Defense for a fourth day to voice their anger about the disqualification of Islamist candidate Hazem Abu Ismael from the presidential election set to start May 23. They also want to disband the presidential election commission.
Abu Ismael was disqualified because of evidence that his late mother had U.S. citizenship, an assertion he has denied, prompting his followers to protest the decision by the election commission.
A day before the attacks, state TV broadcast videos showing protesters chanting against the Supreme Council of the Armed Forces, which has ruled Egypt since President Hosni Mubarak was forced from power last year.
"Dozens of military men dressed in plainclothes started pelting us with stones, cement blocks, and fired tear gas from rifles, so they were obviously security officers under cover," said Alaa Younis, who took part in the sit-in with some friends.
"We fought back with rocks until we noticed they escalated and fired birdshot. Many of us took refuge at one of two field hospitals." Two other presidential candidates announced Wednesday they were temporarily suspending campaign activities because of the violence.
Leading candidate Abdel Monein Aboul Fetftouh, an independent, said on his Twitter account that lawmakers must push the interior minister to investigate. The candidate for the Freedom and Justice Party, Mohamed Mursi, said he is suspending his campaign for two days to "mourn the souls of the martyrs" in the clashes, and blamed the military government for what he called a "massacre."
A government official denied reports that the military was involved. "There were no riot police or military that tried to evict the sit-in by force," said Gen. Marwan Mustapha, a spokesman for the Interior Ministry. "We are not sure of the identities of the thugs that attacked the peaceful protesters. The military only protects the vicinity of the ministry and blocked some roads leading to it."
The epicenter of the violence was around a mosque in Cairo, but clashes continued in several streets. Residents formed neighborhood watches to protect their streets as gunfire crackled for hours, according to witnesses. The military blocked some highways leading to the ministry and did not interfere in the fighting between protesters and the unidentified attackers.
Clashes have erupted in Egypt since an uprising led to the toppling of Mubarak in February of last year, with protesters demanding the military leaders who took over hand over power to a civilian administration. The latest violence comes a few weeks before the presidential elections. About 10 of the 23 presidential contenders have been disqualified, the head of the election committee said earlier this month.
The May 23-24 vote will be the first presidential election since Mubarak's ouster. It comes amid rising political tensions as officials work to craft a new constitution and Egyptians await the June 2 verdict in Mubarak's murder trial.