Egypt’s state-run television said 22 people had been killed and 35 injured in a blast, citing health ministry officials.
Local media reported the explosion hit St. Peter’s Church next to the cathedral, within the same complex.
Photos and video footage showed damage to the exterior of the church, with shattered windows and broken roofing, BBC reported.
The cause of the blast was not immediately clear.
Footage from the scene showed exterior damage following the blast.
The explosion happened at around 10:00 local time (0800 GMT). Local media reported that Cairo’s security chief, Khalid Abdel Aal, arrived to inspect the scene.
Egypt’s Coptic Christians make up about 10% of the country’s population.
St. Mark’s Cathedral is the headquarters of the country’s Orthodox Church, and the home of its leader, Pope Tawadros II.
On Saturday, six policemen were killed when a bomb exploded on a main road leading to the pyramids at Giza. The explosion, at a police checkpoint, was the deadliest attack on security forces in Cairo in over six months.
A recently formed militant group called Hasm claimed the attack.
Two people were killed outside St. Mark’s Cathedral in 2013 when people mourning the death of four Coptic Christians killed in religious violence clashed with local residents.
