Up to 19 people have been reported killed in clashes with Christians protesting against an attack on a church and military police in Cairo.
Protesters threw rocks and petrol bombs at police, and set fire to vehicles, live television showed, in some of the most violent scenes since an uprising overthrew former President Hosni Mubarak in February.
Hundreds from both sides beat each other with sticks on a Cairo bridge, witnesses said.
The country's army rulers have imposed a curfew on the Tahrir Square and downtown area of the city.
State television reported 30 soldiers were injured in the fighting. Coptic Christian protesters said they had been attacked by military police.
Tensions between Christians and Muslims have increased since the February uprising. A similar demonstration last Tuesday ended in the violent dispersal of protesters.
Christians, who make up 10% of Egypt's roughly 80 million people, blamed Muslim radicals for partially demolishing a church in Aswan province last week.
They took to the streets demanding the sacking of the province's governor for failing to protect the building.
Thousands of Christians protested in Cairo and Alexandria over the attack, chanting against the ruling military council and its head, Field Marshal Mohamed Hussein Tantawi.
The protesters want the government to fire the governor of Aswan Province, Mostafa al-Sayed, after the partial demolition of the church on Friday.
Egyptian media said Muslims were accused of attacking the church after talk spread in the town that the building did not have legal authorisation.
