Abdel Fattah al-Sisi spoke on state television as the country came to terms with the worst terror attacks in its recent history after a bomb planted at a mosque in the Sinai town of Bir al-Abd, near Arish, was detonated during Friday prayers.
Al-Sisi said Egyptian security forces would avenge the deaths, restore stability in the region and "respond to the act with brutal force."
"All this is an attempt to stop our efforts in facing terrorism, to destroy our will and our action to stop the terrible criminal plot aimed at destroying what remains of our region," he said. "We will continue. As I said, this attack will only increase our determination ... The armed forces and civilian police will avenge our martyrs and restore security and stability with maximum force over the next few days," the president added.
Three days of national mourning have been declared.
A security official told EFE that an unknown assailant detonated the explosives when the mosque was full of worshippers for Friday prayers. In the wake of the blast, gunmen opened fire on those seeking to escape the building, which was a place of worship for Sufi Muslims, a sect of Islam that has been the target of persecution in the past.
Photographs made available by EPA showed bloodied bodies laid out on the floor of the mosque as emergency responders scrambled to bring the situation under control.
Ambulances that arrived in the immediate aftermath of the blast also came under fire. An investigation into the incident has been opened by Prosecutor-General Nabil Ahmed Sadek. No terror group has so far come forward to claim responsibility for the attack. Several militant groups are active in that area of Sinai, including Al-Wilayat Sinai, which has pledged allegiance to Islamic State and has claimed responsibility for the bloodiest attacks in the region in recent years.