An explosion inside Cairo's Coptic cathedral killed at least 25 people, most of them women, and injured 49, Egypt's state television said.
Security sources told Reuters that at least six children were among the dead. Police and armored vehicles filled the streets surrounding the church and tens of protesters have gathered in front of the church to call for retribution.
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 (26 pounds) of the explosive TNT had denoted on the women's side of the cathedral, the security sources said.
"As long as Egyptian blood is cheap, down down with any president.. the people demand the downfall of the regime" chanted the protesters," protesters cried outside church, using the signature chant of the 2011 revolt against then-president Hosni Mubarak, who later stepped down.
Egypt's presidential office described in a statement the attack as an act of "terrorism" and declared a state of mourning for three days across the country starting today. Also, the government and Al-Azhar condemned, the attack.
We will not allow the terrorist to threaten our national unity with Muslims, Hani Bakhoum, undersecretary of the Coptic Catholic Patriarchate told state television. "We will protect our national unity," he added.
Orthodox Copts, who make up about 10 percent of Egypt's 90 million people, are the Middle East's biggest Christian community. They have long complained of discrimination under successive Egyptian leaders. "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.
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.