Palm Sunday has turned out bloody for Egypt's Coptic Christians as 21 of them were bombed to death in a church in the Egyptian Nile Delta city of Tanta.
About 50 people were reportedly injured, state television said on Sunday.
No immediate claim of responsibility.
The explosive device was planted under a seat at St. George (Mar Girgis) Coptic church, where it detonated in the main prayer hall.
The attack came just one week before Coptic Easter and the same month as Pope Francis is scheduled to visit Egypt.
"Terrorism hits Egypt again, this time on Palm Sunday. Another obnoxious but failed attempt against all Egyptians," the foreign affairs ministry tweeted.
Palm Sunday is the Sunday before Easter and marks the start of Holy Week for Christians.
CNN reported that social media video showed crowds gathered outside the church shortly after the attack.
Copts face persecution and discrimination that has spiked since the toppling of Hosni Mubarak's regime in 2011.
Dozens have been killed in sectarian clashes.
In December, an attack at a Coptic church in Cairo killed 25 people.