Egypt’s President Abdul Fattah al-Sisi has vowed to respond with “the utmost force” after 300 people were killed at a North Sinai mosque during Friday prayers.
The al-Rawda mosque in the town of Bir al-Abed was bombed and fleeing worshippers were then gunned down.
The Egyptian military has said it has conducted air strikes on “terrorist” targets in response.
No group has yet claimed the attack, the deadliest in recent memory.
After bombs were set off, dozens of gunmen waiting outside the mosque opened fire on those trying to escape.
The assailants reportedly set parked vehicles on fire in the vicinity to block off access to the building, and fired on ambulances trying to help victims.
Thirty children were among the dead and at least 100 people have been wounded.
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“What is happening is an attempt to stop us from our efforts in the fight against terrorism,” Mr Sisi said in a televised address hours after the attack.
“The armed forces and the police will avenge our martyrs and restore security and stability with the utmost force.”
Egyptian security forces have for years been fighting an Islamist insurgency in the Sinai Peninsula, and militants affiliated with so-called Islamic State (IS) have been behind scores of deadly attacks in the desert region.
Who was targeted?
Locals say the al-Rawda mosque is used by the local Sawarka tribe, which is known to cooperate with the security services against militants.
It is also known to be popular with worshippers of Sufism, a mystical branch of Islam that is condemned by some jihadist groups.
The head of IS’s “religious police” in Sinai said last December that Sufis who did not “repent” would be killed, after the group beheaded two elderly men reported to be Sufi clerics.
The number of victims is unprecedented for an attack of this type, says the BBC’s Sally Nabil in Cairo. She adds that this is the first time that worshippers inside a mosque have been targeted by militants in North Sinai.
Which militants operate in the area?
Militant Islamists stepped up attacks in Sinai after Egypt’s military overthrew Islamist President Mohammed Morsi following mass anti-government protests in July 2013.
Hundreds of police, soldiers and civilians have been killed since then, mostly in attacks carried out by the Sinai Province group, which is affiliated to IS.
Sinai Province has also carried out deadly attacks against Egypt’s Coptic Christian minority elsewhere in the country, and said it was behind the bombing of a Russian plane carrying tourists in Sinai in 2015, killing 224 people on board.
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