Bodies lie among blood-stained pews in images of one of the attacks – the latest on Egypt’s Christian minority.
Bomb attacks that killed at least 36 people marking Palm Sunday at two Egyptian churches have been claimed by Islamic State.
The first explosion in Tanta, about 70 miles north of Cairo, killed at least 25 worshippers at the Mar Girgis Coptic Church.
Officials said the blast happened “in the front rows, near the altar, during the mass” and that dozens more had been injured.
Video and pictures show crowds gathered around bodies and large blood stains on the pews and church walls.
Several hours later, a bombing in front of Saint Mark’s Church in the coastal city of Alexandria killed at least 11 people and injured many more, according to the health ministry.
Pope Tawadros II, head of the Coptic church, had earlier attended mass there.
Islamic State claimed responsibility for the bombings through its Amaq news agency.
Speaking to Nile TV about the Tanta blast, governor Ahmad Deif said: “Either a bomb was planted or someone blew himself up.”
A witness told the Reuters news agency: “There was a huge explosion in the hall. Fire and smoke filled the room and the injuries were extremely severe.