Egyptian security forces used live ammunition on October 6 to disperse supporters of ousted president Mohamed Morsi who were also attacked by men with firearms and swords, Amnesty International said Monday.
At least 49 people were killed and hundreds wounded in Cairo alone as security forces used "excessive and unwarranted lethal force" to disperse pro-Morsi crowds, said the human rights watchdog.
The Egyptian health ministry said at least 57 people were killed nationwide on the day of the bloody crackdown, including 48 in the capital.
Quoting witnesses, Amnesty said that in some instances, security forces stood by as men in civilian clothing used firearms, knives and swords to attack the Islamist demonstrators.
On October 6, security forces clashed with pro-Morsi crowds in Cairo as they tried to reach the capital's Tahrir Square where supporters of the military were celebrating the 40th anniversary of the Arab-Israeli war.
"The Egyptian security forces patently failed to prevent the loss of life. In a number of cases bystanders or non-violent protesters were caught up in the violence," Amnesty's Hassiba Hadj Sahraoui said in a statement.
"Although some pro-Morsi protesters threw rocks, burned tyres and used fireworks or other incendiaries against security forces and local residents, the security forces... resorted to the use of lethal force when it was not strictly necessary."
Amnesty said no members of the security forces were killed during the clashes.
The clashes were the deadliest since security forces stormed two Cairo protest camps of Morsi supporters on August 14, leaving hundreds of people dead.
Amnesty said the worst bloodshed was in the Cairo district of Dokki, where 27 people died as a result of live rounds and three from shotgun pellet wounds.
At Dokki, the security forces used "teargas, shotguns and live ammunition against protesters attempting to reach and cross a bridge leading to Tahrir Square," said the London-based watchdog.
Amnesty said another 16 people were "shot dead" near Ramses Square when security forces used live ammunition to disperse protesters trying to reach Tahrir.
"One bullet went straight through me and hit the man standing behind me," a 16-year-old schoolboy who was shot in the arm and leg was quoted as saying by the watchdog.
"There was heavy teargas lingering in the air, and bullets whizzing by...people were running away, and security forces were chasing them... We ran with the crowd, people were falling around us," said Umm Sara, another pro-Morsi supporter.
Calling for a full and impartial investigation, Hadj Sahraoui said the "Egyptian security forces have an abysmal track record of using disproportionate force during protests."
"The authorities' utter disregard for international standards on the lawful use of force suggests that they are prepared to crackdown on Morsi supporters at any cost."
Egypt's military-installed authorities have been waging a campaign to crush Islamist supporters of Morsi since the storming of the two Cairo camps on August 14.
More than 1,000 people have been killed in the crackdown, and 2,000 Islamists have been detained. Morsi himself is being held at an unknown location after the army toppled him on July 3.