At least 36 people detained on suspicion of taking part in the street clashes roiling Egypt's capital were killed when security forces fired tear gas inside of the prison truck holding them, security officials said.
Those killed were part of a prison truck convoy of some 600 detainees heading to Abu Zaabal prison in northern Egypt, the officials told The Associated Press. Detainees in one of the trucks rioted and managed to capture a police officer inside, the officials said.
Security forces fired tear gas into the truck in hopes of freeing the badly beaten officer inside, the officials said. The officials said those killed died from suffocating on the gas.
The officials spoke on condition of anonymity because they were not authorised to speak to journalists.
However, the officials' version of events contradicted reports about the incident carried by state media. The official website of Egyptian state television reported that the deaths took place after security forces clashed with militants near the prison and detainees came under fire while trying to escape. The official MENA state news agency also said the trucks came under attack from gunmen.
State media also said all those killed and the gunmen belonged to the Muslim Brotherhood, the organisation that ousted President Mohammed Morsi hails from.
The officials who spoke to AP said some of the detainees belonged to the Brotherhood, while others didn't.
The differences in the accounts could not be immediately reconciled Sunday night.
Meanwhile supporters of ousted president Mohamed Morsi cancelled some Cairo marches today for "security reasons", as the military chief vowed to face down violent protests following Egypt's bloodiest week in decades.
The latest developments come as senior European Union diplomats were to hold emergency talks Tuesday in Brussels to discuss the situation in Egypt and any future EU action.
More than 750 people were killed in four days of violence after the military and police launched a blistering crackdown on Islamist protest camps, sparking international condemnation.
In his first remarks since the campaign began on Wednesday, military chief Abdel Fattah al-Sisi said security forces would confront further violence from protesters.
"We will never be silent in the face of the destruction of the country," Sisi told top military and police commanders.
"We are very prepared for this," he said, and pledged a "forceful" response to further attacks on police stations and government buildings.
The Islamists said they cancelled "several marches", citing fears of vigilantes and snipers but that others would go ahead.
At the site of at least one planned march, residents gathered vowing to forcibly prevent any demonstration.
"Several marches in Cairo have been cancelled for security reasons," said Yasmine Adel, a spokeswoman for the Anti-Coup Coalition of Morsi loyalists.
A separate statement by the group said "all nationwide marches and Cairo marches are continuing as planned" with the exception of a rally in the Roxy area of the capital.
Several hundred protesters briefly marched in the Suez canal city of Ismailiya, an AFP correspondent said.
The Islamist coalition has grown increasingly disorganised as some leaders have been arrested and others are in hiding.
At a mosque in the Dokki neighbourhood, where one march was scheduled to begin, residents stood guard.
"We are waiting for them. I swear we will kill them if they approach the mosque," one said.
The rallies had been expected to be a test of the strength of Morsi's loyalists after four days of deadly clashes with police.
On Saturday, police stormed a Cairo mosque where Islamists were holed up, after trading fire with gunmen inside its minaret.
On Sunday morning, the capital showed signs of returning to normal, with traffic beginning to flow again and banks and shops reopening for the first time since Wednesday.
That was when security forces stormed two camps of Morsi supporters, sparking clashes that killed at least 578 people across the country.
In the 24 hours after that, another 173 people were killed, according to the government, putting the four-day toll at 751.
The violence has shocked the international community, but Egypt's government - installed by the army after Morsi's July 3 ouster - has fiercely defended its actions.
The unrest, including a crackdown on Morsi's Muslim Brotherhood and the arrest of top Islamist leaders, has left Egypt divided as never before.
Vigilantes have pulled bearded men out of cars, and a mob chanted "terrorists" as Morsi supporters were dragged from Cairo's Al-Fath mosque on Saturday.
According to an AFP tally, more than 1,000 people have been killed since mass demonstrations against Morsi at the end of June, among them a son of the Brotherhood's supreme guide on Friday.
The government, meanwhile, dismissed reports that it could ban the Brotherhood.
"We're not into the effort of dissolving anyone or preventing anyone" from taking part in politics, presidential adviser Mustafa Hegazy said on Saturday.
Violence has also continued to plague the Sinai Peninsula, where a civilian, two soldiers and a policeman were killed overnight Saturday, security sources said.
The European Union said Sunday it would review ties with Egypt's army and government unless the bloodshed ends.
"The EU will urgently review in the coming days its relations with Egypt," EU leaders Herman Van Rompuy and Jose Manuel Barroso said.
British Foreign Secretary William Hague called his Egyptian counterpart Saturday to condemn violence by both sides, and described as "unacceptable" attacks against churches.
The United States has announced the cancellation of its biannual military exercise with Egypt, and its embassy in Cairo was closed for security reasons.
The White House has stopped short of suspending $1.3 billion in annual aid, although some US lawmakers called Sunday for the funds to be cut.
But the international response has not been uniformly critical. Both Saudi Arabia and Jordan have said they back Egypt in its fight against "terrorism".