Cairo: An explosion inside Cairo's Coptic cathedral on Sunday killed at least 25 people – most of them women – and injured almost 50 others.

Security sources said that at least six children were among the dead inside St Mark's Coptic Orthodox Cathedral.

Police and armoured vehicles filled the streets surrounding the church, as protesters 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.

A device containing around 12 kilograms of the explosive TNT had detonated 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 the attack as an act of terrorism and declared a state of mourning for three days across the country. 

Hani Bakhoum, undersecretary of the Coptic Catholic Patriarchate, told state television his church would not allow the blast to divide Egyptians. 

"We will not allow the terrorist to threaten our national unity with Muslims," Hani Bakhoum said. "We will protect our national unity."

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.

"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 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.

Reuters