According to reports, at least 10 Copts and one Muslim were killed during clashes in the Al-Khosous district of Qaliubia province.
Violence broke out after swastikas were drawn on the side of a mosque wall which local Muslims alleged had been done by Christian children.
According to AINA news agency, Father Sourial Younan, the priest of St Georges Church,said the Copts were shot dead in front of the church.
Then on Sunday, stones were thrown and gunshots fired at mourners as they gathered for the funerals of the four victims at St Mark's Cathedral, Cairo.
According to reports, the Christians came under fire as they exited the cathedral. Some Muslims were on the roofs of neighbouring buildings where they hurled stones and shot at the Christians. Some Christians retaliated by throwing stones back.
One Copt, Mahrous Hanna Ibrahim, reportedly died from gunshot wounds to the head and neck. Another 24 Copts were reportedly injured.
It is understood police were slow to arrive and then proceeded to fire tear gas into the cathedral where many of the Copts had taken refuge.
In a statement, UK Coptic leader His Grace Bishop Angaelos underlined the lack of "intentional, pragmatic and proactive leadership" from the Egyptian state, saying police had done "very little" to prevent the attacks.
He expressed concerns about the growing violence in Egypt, particularly against Christian communities.
"It is now clear that the state needs to take that responsibility far more seriously," he said.
Bishop Angaelos lamented the contrast to "the collective pride, passion and faithfulness of Egypt" that had been displayed when Christian and Muslim communities gathered together in the same cathedral in 2011 to pay tribute at the departure of the late Pope Shenouda III.
The Council of Churches of Egypt condemned the attack as unacceptable. Pope Tawadros II called for calm and urged all citizens to "maintain the safety of the nation, the lives and national unity".