* Rare official criticism of security forces
* Bloodiest day in modern Egyptian history
* Crackdown widens (Adds quotes, details)
By Maggie Fick and Shadia Nasralla
CAIRO, March 5 (Reuters) - A government-appointed panel said on Wednesday that the deaths of hundreds of Muslim Botherhood supporters at a protest camp in Cairo last August was mostly the fault of demonstrators who had provoked the security forces into opening fire.
It found that 632 people were killed, 624 of them civilians in one of the bloodiest days in Egypt's modern history.
But the protesters had brought it upon themselves as armed men within their ranks had shot first at the security forces and also used civilians as human shields, it said.
The findings mainly echoed the military-backed government's version of events. But in an unusual move, the panel also placed some responsibility for the bloodshed on the security forces and said they had used disproptionate force.
The mass killings took place when the security forces moved to dismantle the protest camps set up by supporters of Islamist President Mohamed Mursi, who was overthrown by the army six weeks earlier after demonstrations against his rule.
Security forces then mounted a harsh crackdown on his Muslim Brotherhood movement.
The commission's findings, announced at a news conference on Wednesday, were the most detailed official account of the dispersal of Brotherhood supporters who had camped around the Rabaa al-Adawiya mosque in northeast Cairo for weeks - a flashpoint in the struggle between the Islamist movement and the new army-backed government.
During a weeks-long standoff, international mediators tried to persuade the government to avoid using force in Rabaa and escalating a political crisis. But hardliners prevailed.
Security forces, including snipers, stormed the camps on Aug. 14, firing live ammunition under the cover of army helicopters. Bulldozers tore down tents which were set ablaze, witnesses said.
Protesters who survived the onslaught said police fired tear gas at children before shooting bullets at demonstrators attempting to flee.
The government called for an investigation after rights groups pressured authorities to set up a fact-finding committee as a first step towards accountability for the killings.
MOSTLY PEACEFUL PROTESTERS
The panel said that in addition to the 632 deaths at Rabaa, 686 protesters were killed in clashes across Egypt in the three days following the violence in Cairo.
But its presentation focused on what it called violations by the pro-Mursi protesters. Panel member Nasser Amin accused the Mursi supporters of detaining and torturing civilians at the protest camps.
He said some protesters also carried arms and shot at security forces, causing them to fire back.
But most of the protesters were peaceful and some had been used as human shields by the gunmen, he said.
Amin also said security forces had contributed to the bloodshed. They had failed to secure safe passage for protesters after clashes erupted and did not give them enough time to flee.
The 25 minutes between warnings on loudspeakers and the assault by the security forces "was not enough for thousands of protesters to leave," he said.
Protesters were deprived of life-saving aid because ambulances were not able to access the conflcit area, he said.
And contradicting past official accounts, Amin said security forces did not maintain proportional use of force when confronted with heavy gunfire from protesters.
The Interior Ministry has said that authorities did not use excessive force to scatter the camps and that Mursi's supporters fired first.
Rabaa has become a symbol of the suppression of the Brotherhood, which has largely been driven underground since then. The government has declared it a terrorist group, arresting thousands of its members and putting Mursi and other leaders on trial.
The Brotherhood had won the vast majority of elections since after a popular uprising backed by the army toppled autocratic president Hosni Mubarak in 2011. (Additional reporting by Noah Browning, Omar Fahmy, and Mahmoud Mourad, Writing by Maggie Fick, Editing by Michael Georgy and Angus MacSwan)