The ruling military council in Egypt has ordered government ministers to carry out a speedy investigation into the killing of 24 people at a rally of Coptic Christians in Cairo on Sunday. Another 200 were injured.
The head of the Church has ordered Copts to begin a three-day fast and pray for peace.
It is the worst outbreak of trouble since President Hosni Mubarak was overthrown in February. At least 40 people have been arrested.
The BBC reports that mourners blame the army for the deaths.
Protesters say they were attacked by thugs before the security forces fired on them and drove military vehicles into the crowds.
The Supreme Council of the Armed Forces has told the government to carry out a swift inquiry.
Copts, who make up about 10% of Egypt's 85 million population, have a number of grievances against the interim administration.
They say the authorities have been slow to punish Islamists who have attacked their churches.
Sunday's protest was sparked by the burning of a church in southern Aswan province last Friday.