Egyptian authorities have arrested dozens of people in the capital, Cairo, after clashes between Coptic Christians and police killed 24 people and wounded at least 200 others.
Officials did not say whether those arrested after Sunday's violence were Christians or Muslims.
Coptic Christians had been protesting the recent burning of a church when the demonstrations escalated into rioting against military rule.
Egypt's interim Cabinet says it will not allow any group to manipulate national unity or delay its democratic transition.
State media reported the Cabinet is holding an emergency session to discuss the situation, and the army imposed a nighttime curfew on Tahrir Square and the surrounding area.
Protesters used firebombs, set army vehicles on fire and battled with security forces in Egypt's most violent clashes since the February uprising that ousted Hosni Mubarak as Egypt's president.
Hundreds of rioters fought with police, some of them tearing up pavement and using rocks for ammunition. At one point, an armored security van sped into the crowd, crushing some protesters to death. Two soldiers also died in Sunday's clashes.
Christian demonstrators said a peaceful rally in the mostly Coptic neighborhood of Shubra turned deadly when the group marched to Egypt's Radio and Television Building where plainclothes police attacked them. The protests later spread to Tahrir Square, the focal point of the February revolution.
In the past few weeks, riots have broken out at two churches in southern Egypt, prompted by Muslim crowds angry about church construction.
One clash took place near the city of Aswan after church officials agreed to a demand by local ultra-conservative Muslims – known as Salafis – that a cross and bells be removed from the church building.
Aswan's governor, Mustafa Kamel al-Sayyed, is also reported to have suggested that the church does not have legal authorization. Protesters said Sunday they are demanding the governor's ouster after the church was burned and partially demolished last week.
Christians make up about one-tenth of Egypt's 80 million people and often face attacks from Islamic extremists. Copts joined with Muslims during the protests that ousted Mr. Mubarak, but sectarian troubles have since intensified.
Egypt has been considering new laws designed to stem sectarian violence, including banning protests at places of worship and the use of religious slogans to incite hatred.
In May, 12 people were killed in sectarian clashes between Christians and Muslims after rumors spread that Christians were holding a woman who had converted to Islam.
