A suicide bomber launched the attack on a church in Alexandria (north) in which 11 people were killed and 35 wounded. The man, carrying a belt of explosives, detonated his cargo after police prevented him from entering St. Mark's Church, the interior ministry said.
Coptic Pope Theodore II, who had accompanied Sunday's celebrations on Sunday, left the temple before the blast, his personal secretary said.
Hours earlier, a first attack left 27 dead and 78 wounded in the church Mar Girgis de Tanta, 120 km north of Cairo, in the Nile delta.
This attack took place at 10 am (5 am) in the interior of the church, "in the first queues near the altar during Mass," General Tarek Atiya, deputy minister of the Interior in charge of press relations, told AFP.
The death toll rose sharply from 13 dead who were first announced to 27 dead and 78 wounded, according to the ministry of health.
Security services inspected the temple grounds to ensure there were no more explosive devices, General Ahmad Deif, Gharbia's governor - whose capital is Tanta - told Nile News television.
According to him, the nature of the attack is not yet known. "Either it was a bomb inside the church, or someone detonated their explosives," he said.
Images posted on the Extra News private television network showed the floor and white walls of the church covered with blood, as well as shattered wooden benches.
The EI claimed both attacks early in the afternoon, months after his Syrian arm called out to attack "the infidels or apostates in Egypt and everywhere", a way of referring to the Coptic community.
- Bloody Attacks -Prime Minister Sherif Islamil condemned Tanta's attack and insisted on "the state's determination to eradicate such terrorist acts and root out terrorism."
Al-Azhar, the prestigious institution of Sunni Islam based in Cairo, also condemned this attack. "The aim of this cowardly terrorist attack is to undermine the security and stability of our Egypt, and the unity of the Egyptian people, which requires all members of society to remain united," he said.
Sunday's blasts took place a few days before a papal visit to Egypt on April 28 and 29. "To my dear brother, your holiness, Pope Theodore II, to the Coptic Church and to the whole beloved Egyptian nation, I express my deep sorrow," said Pope Francis during the Angelus prayer after learning what had happened.
On 11 December, an EI suicide killed 29 people in the Coptic church of St. Peter and St. Paul in Cairo.
With the Cairo bombing, there have been repeated calls to tighten the fight against the extremist movement in Egypt, especially in Sinai, where it carried out a series of bloody attacks against the security forces.
The Egyptian army announced on April 2 that it had killed Abu Anas al-Ansari, one of the founding members of the EI's local arm, Ansar Beit al Maqdes, in a bombing.
This group had claimed a bomb attack on a Russian airliner that crashed on 31 October 2015 with 224 people on board after taking off from the Egyptian resort of Shark el Sheikh.
The Orthodox Copts of Egypt represent the largest Christian community in the Middle East and one of the oldest. Its members claim to be victims of discrimination throughout the country by the authorities and the Muslim majority.