One person was killed on Sunday in clashes at Cairo's Coptic cathedral, after funeral prayers for four Christians during which angry Copts chanted against Islamist President Mohammed Morsi, an official said, according to AFP.
Fighting between Christians and Muslims, meanwhile, erupted anew in a town north of the capital where sectarian violence killed five people on Friday, including the four Coptic Christians, police said.
The bitter clashes underscore the simmering tensions in a divided Egypt that has seen violent confrontations between Morsi's main Islamist allies and a wide-ranging opposition.
They also highlight sectarian tensions that have been brewing for several years.
Egypt's Christians, who make up six to 10 percent of the country's population of 82 million, have regularly complained of discrimination and marginalization.
There have been several attacks, some of them lethal, against Copts in Egypt. Last year, Muslims attacked a Coptic church in a village near Cairo. At least 16 people were wounded in the melee, among them 10 police officers.
In one case, two Coptic Christian children, aged 10 and 9, were arrested for insulting religion, after an imam accused them of tearing up pages of the Koran.
Morsi, in a call to Coptic Pope Tawadros II, late on Sunday condemned the violence.
He ordered "an immediate investigation" into the clashes and condemned the clashes at the cathedral as "an attack on myself", according to a statement published by the official MENA news agency.
He also affirmed that "the protection of all citizens, Muslims and Christians, is the responsibility of the state."
Witnesses to the Cairo clashes said they began when mourners were pelted with stones by residents of the area as they left the cathedral, symbol of the Coptic community.
Black-clad riot police intervened, firing tear gas at the cathedral, witnesses said, but not before one person had been killed.
There were scenes of chaos outside the cathedral in the central Cairo neighborhood of Abbassiya where Coptic bishops had been calling for peace and calm after the killings on Friday.
Loud blasts could be heard, as clouds of smoke rose up into the sky and people ran in several directions, reported AFP.
Rows of Abbassiya residents hurled rocks and bottles at the cathedral and were met in kind from Copts inside the church complex.
The mourners had been planning to carry the bodies of the Christians out of the cathedral to the presidential palace as a protest against the violence, one of them said.
Sunday's service was being held for the four Christians killed in the sectarian clashes two days earlier.