Cairo: An explosion on Sunday inside Cairo's Coptic cathedral killed at least 20 people and injured 35, according to Egyptian state television, in the second deadly attack to hit the city in two days.

Egypt's official MENA news agency said an assailant lobbed a bomb into a chapel on Sunday close to the outer wall of St Mark's Cathedral, seat of Egypt's Orthodox Christian church and home to the office of its spiritual leader, Pope Tawadros II.

There was no immediate claim of responsibility.

Egyptian security forces are fighting an Islamist insurgency led by Islamic State's branch in North Sinai, where hundreds of soldiers and police have been killed. 

On Friday, two roadside bombs - one in Cairo and one north of the capital - killed six policemen and wounded six others.

More to come.

With Reuters & AAP