Egypt's official news agency says 36 people were killed when Muslim Brotherhood detainees tried to escape from a prison truck convoy in northern Cairo.
The agency reported that gunmen - who were also Muslim Brotherhood supporters - attacked a convoy of trucks carrying more than 600 detainees, but did not explain how the deaths occurred.
The detainees were reportedly being transferred to Abu Zaabal prison in northern Cairo when the attack happened.
There are conflicting accounts as to how some 38 supporters of the Muslim Brotherhood died in an incident involving Egyptian prison vans.
The Interior Ministry said that the detainees tried to escape from the vans and took a prison guard hostage. It added that an undisclosed number of people had died from inhaling tear gas rounds.
But a legal source told Reuters that the Brotherhood followers suffocated in the back of a crammed police van on the way to prison.
Some 38 supporters of the Muslim Brotherhood died on Sunday in a riot at an Egyptian prison, security sources said.
The Interior Ministry did not immediately confirm the exact death toll, but said in a statement that a number of detainees had died after trying to escape from a prison.
Egypt's most senior military leader warned today that the army will not stand by if political violence in the country continues.
General Abdel-Fattah al-Sisi was making his first comments since Wednesday's crackdown on Muslim Brotherhood protesters in the capital Cairo.
It also emerged that the death toll from violence in the last few days has reached 830.
ITV News' International Correspondent John Irvine reports from Cairo:
Some 38 supporters of the Muslim Brotherhood died today in an incident in an Egyptian prison, Reuters reports citing security sources.
Egypt's state news agency reports that the death toll from violence across the country in the last few days has risen to 830.
- Wednesday and Thursday: 578
- Friday: 173
- Saturday: 79
Egypt's army chief Abdel Fattah el-Sisi has told supporters of the ousted president Mohamed Morsi that there is "room for everyone in Egypt".
In a statement on the army's Facebook page, he said that the military has no intention of seizing power, but that it will not stand by silently in the face of violence.
The Egyptian army chief Abdel Fattah el-Sisi has told Mohamed Morsi supporters "there is room for everyone in Egypt", Reuters reports citing the army's Facebook page.
The deaths in Egypt cannot be "justified nor condoned" the European Union has said.
In statement European Commission President José Manuel Barroso and European Council President Herman Van Rompuy said:
Recent developments in Egypt, and more particularly the violence of the last days, are extremely worrying. We are distressed at the loss of lives and we offer our sincere condolences to the families of the victims.
Egypt is a key partner of the European Union. We share interests in and responsibility for the maintenance of peace and stability in a strategic region. Further escalation must be prevented. It could have unpredictable consequences for Egypt and for its broader neighbourhood.
It is crucial that violence ends immediately.
While all should exert maximum restraint, we underline the particular responsibility of the interim authorities and of the army in bringing clashes to a halt. The violence and the killings of these last days cannot be justified nor condoned. Human rights must be respected and upheld. Political prisoners should be released.
There is no alternative to dialogue.
The European Union said in the coming days it will review its relations with Egypt and adopt measures to pursue these goals, a statement said.