300 people were killed at a North Sinai mosque in Egypt 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 are among the dead and at least 100 people have been wounded.
“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.