An explosion inside Cairo's Coptic cathedral killed at least 25 people, most of them women, and injured 49, Egypt's state television says.
Security sources told Reuters that at least six children were among the dead.
There was no immediate claim of responsibility, however, some supporters of the Islamic State militant group celebrated the attack on social media.
"God is great, God is great, God is great," one wrote on Telegram messenger.
"God bless the person who did this blessed act," wrote another, also on Telegram.
A device containing about 12 kg of the explosive TNT had denoted on the women's side of the cathedral, the security sources said.
"As soon as the priest called us to prepare for prayer, the explosion happened," Emad Shoukry, who was inside the cathedral when the blast took place, told Reuters.
"The explosion shook the place... The dust covered the hall and I was looking for the door, although I couldn't see anything... I managed to leave in the middle of screams and there were a lot of people thrown on the ground," he said.
Orthodox Copts, who make up about 10 per cent of Egypt's 90 million people, are the Middle East's biggest Christian community.
They have long complained of discrimination under successive Egyptian leaders.
Islamists are waging an insurgency led by Islamic State's branch in North Sinai, where hundreds of soldiers and police have been killed.
The insurgents have also launched deadly attacks Cairo and other cities.
On Friday, two roadside bombs - one in Cairo and one north of the capital - killed six policemen and wounded six others.