CAIRO—A bomb exploded at Cairo’s main Coptic Christian cathedral compound on Sunday morning, killing at least 25 people and wounding another 49, in the largest attack on a Christian house of worship in Egypt since 2011.

The blast went off on the women’s side of the worshiping hall in the small church of St. Peter and St. Paul, attached to the Coptic cathedral in the capital’s Abassyia district, state media reported.

No group has claimed responsibility for the attack, which was quickly condemned by the Egyptian government and the head of Cairo’s Al-Azhar mosque, the seat of Sunni Islamic learning and one of the world’s oldest institutions of religious teaching.

Survivors described the church as being packed with worshipers during a national holiday to celebrate the anniversary of the Prophet Muhammad’s birth. The blast shattered the silence of the hall as those gathered listened to a sermon partly honoring a deceased church member.

“The turnout was bigger than normal,” said Tahani Gabriel, 65 years old, who sat in the third row of the church. Her cousin Souad Atta, the widow of the man mourned, was in the first row and was killed, Ms. Gabriel said.