The interior ministry said the explosion was in Mergerges church in Tanta, a city in the north of Egypt in the Nile Delta, located between Cairo and Alexandria.
The church was full at the time with worshippers observing Coptic Christian Palm Sunday.
Egyptian state television spoke with the adviser to the health minister for emergency, who said 21 people died and 59 people were injured. The director of the provincial ambulance service Magdi Awad also confirmed to The Associated Press that 21 people died and many more wounded.
Christians make up roughly 10 percent of Egypt’s population, and nearly all Christians are Copts, of the Coptic Orthodox Church.
Christians in Egypt have been targeted before in attacks by Islamic extremists. In December, 25 people died when a bomb exploded during Sunday mass in Cairo, as NPR’s Bill Chappell reported. At the time it was “the deadliest attack on Egypt’s Christian minority in years,” he notes.
ISIS has targeted Coptic Christians. The group released a video two years ago appearing to show a mass beheading of 21 Egyptian Coptic Christians in Libya.
And on New Year’s Day in 2011, a bombing at a church in Alexandria killed at least 21 people, as NPR’s Jane Arraf noted.
No group has yet claimed responsibility for the attack.