Egypt's Health Ministry said an explosion at a church in the coastal city of Alexandria killed 17 people and wounded at least 48 others.
It was the second attack targeting Egypt's Coptic Christians after a bomb in a church in the Nile Delta town of Tanta killed 27 people and wounded more than 78.
The ministry said the explosion went off at Saint Mark's Church in Alexandria, where Pope Tawadros II had earlier celebrated Palm Sunday.
Islamic extremists have repeatedly targeted Egypt's Christian minority in the past.
An Islamic State affiliate based in the Sinai Peninsula claimed an attack on a Cairo church in December that killed around 30 people and vowed to carry out more attacks on Christians.
IS claimed the attacks via its Aamaq news agency, after having recently warned that it would step up attacks on Egypt's Christians.
The blasts came at the start of Holy Week leading up to Easter, and just weeks before Pope Francis is due to visit the Arab world's most populous country.
CBC TV showed footage from inside the church in Tanta, where a large number of people gathered around bodies covered with papers.
Pope Francis condemned the bombings, expressing "deep condolences to my brother, Pope Tawadros II, the Coptic church and all of the dear Egyptian nation". Word of the attacks came as Francis himself was marking Palm Sunday in St Peter's Square.
Grand Sheikh Ahmed el-Tayeb, head of Egypt's Al-Azhar - the leading centre of learning in Sunni Islam - also condemned the attacks, calling them a "despicable terrorist bombing that targeted the lives of innocents".
Both Israel and the Islamic Hamas movement ruling neighbouring Gaza also condemned the bombings.
The bombings add to fears that Islamic extremists who have long been battling security forces in the Sinai Peninsula are shifting their focus to civilians.
Turkey condemned the attacks.
Presidential spokesman Ibrahim Kalin tweeted his condolences, saying: "We strongly condemn the heinous terror attacks on churches in Egypt on Palm Sunday today."
Mehmet Gormez, the head of religious affairs in Turkey, "cursed" the attacks and said they are the shared problem of all humanity.
"The immunity of a place of worship, no matter the religion it belongs to, cannot be violated and the bloodthirsty killing of innocent worshippers cannot ever be forgiven," said Mr Gormez.
Turkey's ministry of foreign affairs also published a statement denouncing the attack in the Nile Delta town of Tanta, which killed at least 26 people.
"We convey our condolences to the bereaved families and the whole people of Egypt," the statement said before a second attack hit the church in Alexandria, killing at least 11 people.
The blast at the church in Alexandria was caused by a suicide bomber who was stopped by police when trying to storm the entrance, said Egypt's interior ministry.
Three policemen were among those killed in the attack.
