At least 20 people have been killed in Cairo in an attack on a protest near the Egyptian Ministry of Defence.
The unknown attackers used rocks, clubs, firebombs and shotguns. The protesters retaliated, beating some assailants.
Soldiers and police have now intervened to stop the clashes, but as long as six hours after the violence started.
Two leading presidential candidates have suspended campaigning in protest at the way authorities handled it.
Abdul Moneim Aboul Fotouh, an independent Islamist, and Mohammed Mursi, head of the Muslim Brotherhood's Freedom and Justice Party (FJP), criticised the authorities' response.
In addition, the FJP and the Salafist Nour party, which together control 70% of the seats in parliament, decided to boycott a meeting with the ruling Supreme Council of Armed Forces (Scaf).
Many of the protesters who were attacked outside the defence ministry building, in the capital's Abbasiya district, on Wednesday morning were supporters of a Salafist preacher barred from standing in the election.
Hazem Abu Ismail was disqualified because his mother had dual Egyptian-US nationality, violating rules laid out in a constitutional declaration approved after an uprising forced President Hosni Mubarak to step down.
Mr Abu Ismail complained that he was the victim of a "plot" by the military authorities, but the election commission found no evidence.
Ahmed Raafat was at the demonstration with friends and witnessed the attacks which he described to the BBC as a "massacre".
"I saw several people with head injuries and two people died in front of me. There was a lot of blood everywhere.
"The thugs who carried out the attacks were on foot and had their faces covered... [They used] live bullets, molotov cocktails, bricks and tear gas," he said.
The health ministry said more than 150 people were injured on Wednesday and that many were receiving treatment at a nearby field clinic.
Some had gunshot wounds and others had been attacked with knives, according to medical sources.
Criticism of military
Some reports said the unidentified attackers were local people, angry at the disruption caused by the sit-in, which began on Saturday.
But many Egyptians will suspect that, at the very least, the government turned a blind eye, reports the BBC's Jon Leyne in Cairo.
The use of plain-clothes thugs is a well-worn tactic by the authorities, he adds.
Soldiers and police deployed in the area initially did not intervene to stop the clashes, nor did they when one person was killed in a similar attack early on Sunday.
The Nobel laureate and former head of the UN's nuclear watchdog, Mohamed ElBaradei, criticised the ruling generals on Twitter .
"Massacre in front of [the ministry of defence]," he wrote. "Scaf & government unable to protect civilians or in cahoots with thugs. Egypt going down the drain."
Opposition to the Scaf has built up steadily since it assumed Mr Mubarak's presidential powers in February 2011.
The council has been accused of stifling dissent by killing protesters, detaining critics and undermining the youth and civil society groups which led the uprising.
In another development, the deputy head of Scaf has said the military might hand over power to an elected president straight after this month's first round of elections, if a candidate wins outright.
General Sami Annan was reported by a state-run Egyptian television channel (Nile News TV) as saying the military was studying the possibility of transferring power on the 24th of May.
They had previously committed to handing over power by the end of June, after a presidential election that they say will be free and fair.
The election's first round is scheduled for 23 and 24 May, with a run-off vote for the top two candidates expected on 16 and 17 June.
The race seems to have narrowed to a contest between Mr Aboul Fotouh, Mr Mursi, and the former head of the Arab League, Amr Moussa.