At least 230 people were killed when terrorists detonated a bomb and gunned down worshippers in the North Sinai region.
The president said that Egypt will stand strong in the face of terrorism, and will continue its war against it.
Three police officers told the AP that militants attacked the al-Rawdah mosque in the town of Bir al-Abd, which is located about 25 miles from the North Sinai provincial capital of el-Arish. Islamic militants, including the local affiliate of the Islamic State group, consider Sufis heretics due to their less literal interpretations of the faith.
The militants opened fire from four off-road vehicles on the hundreds of worshippers attending the sermon in the mosque.
The global community must continue to strengthen its efforts to defeat terrorist groups that threaten the United States and our partners and we must collectively discredit the extremist ideology that forms the basis of their existence.
The gunmen also fired at ambulances transferring the injured to hospital, eyewitnesses said.
El-Sisi said that the military and police "will avenge our martyrs" and will respond "using brute force against those fragments of terrorists" in the coming period to ensure stability and security.
Sisi has support from some Bedouin tribal leaders, who have helped the army locate weapon-smuggling routes used by jihadi groups, security officials said.
Bloodshed in the Sinai worsened after 2013 when Sisi led the overthrow of President Mohamed Mursi of the Muslim Brotherhood.
As of Friday afternoon, no terror group had claimed responsibility for the attack.
Hundreds of police, soldiers and civilians have been killed since then, a lot of them in attacks carried out by fighters loyal to Islamic State, who consider Sinai one of the provinces of the group's self-proclaimed caliphate.
Militants have tried to expand their operations into Egypt's heavily populated mainland, hitting Coptic Christian churches and pilgrims.