Worshippers at three Christian churches came under attack from knife-wielding assailants during Mass Friday.
Police said one worshipper was killed and more than a dozen wounded in the simultaneous attacks in the northern city of Alexandria.
Police were searching for three men, one in each attack.
Hundreds of Christians gathered in angry protest outside the Coptic Christian churches, and witnesses said clashes erupted between Christians and Muslims.
Initial police reports said a total of 17 people were injured: 10 at the Saints Church in downtown Alexandria and three at the nearby Mar Girgis Church. A third attacker wounded four worshippers at a church in Abu Qir, a few miles to the east.
One worshipper was killed and at least two others were in serious condition, a police official said on condition of anonymity because he was not authorized to speak to the press.
The attack comes on what is Good Friday to many of the world's Christians. However, Egypt's Coptic Christians, and other followers of the Greek Orthodox church, celebrate the holiday a week later.
Coptic Christians account for about 10 percent of Egypt's population of 72 million and generally live in harmony with the Muslim majority, though violence flares occasionally.
In October, Muslims attacked churches and shops in Alexandria over the distribution of a DVD of a play deemed offensive to their religion. Four people were killed in weeklong riots.