At least 54 Egyptian police officers were killed when a raid on a militant hideout outside of Cairo escalated into an all-out firefight, authorities said.
It was one of the largest losses of life for Egyptian security forces since militants began targeting government forces after the Islamist president Mohammed Morsi was removed by the military in 2013.
The gunfight began late on Friday in the al-Wahat al-Bahriya area in the Giza region, about 84 miles from the capital, after security services moved in.
The officials said the death toll could increase.
Police also deployed aircraft to confront the militants, officials said, as clashes continued after nightfall.
No militant group immediately claimed involvement in the firefight.
Egypt has been struggling to contain an insurgency by Islamic militants led by an affiliate of the Islamic State group, centred mostly across the Suez Canal in the northern region of the Sinai Peninsula. Attacks on the mainland have also recently increased.
The country has been under a state of emergency since bombings and suicide attacks targeting minority Coptic Christians killed scores earlier this year. Those attacks were claimed by IS.
Egypt’s Grand Mufti Shawki Allam condemned the killing in a statement on Friday.