Suspected militants attacked two police minibuses with rocket-propelled grenades, killing 24 officers, Egyptian officials say.
The ambush, condemned by Egypt's interior ministry as the work of "armed terrorist groups", took place as the two vehicles were driving through a village near the border town of Rafah in the volatile Sinai Peninsula.
Two officials, who spoke only on condition of anonymity, said that three officers had also been injured in the attack.
Sinai has been witnessing near daily attacks by suspected militants since the Islamist president Mohammed Morsi was ousted on July 3.
It is estimated nearly 50 security officials have been killed in the region since then, and the army claims to have killed nearly 70 "terrorists" in the same time.

The van was attacked with rocket-propelled granades
In response to the attack Egypt closed the Rafah crossing, a border official has confirmed.
The latest attack follows the brutal suppression of Mr Morsi's supporters in Cairo in scenes of bloodshed over the last five days, which have left at least 750 people dead.
The military, which assumed control of the country from Mr Morsi, has been rounding up the former president's Muslim Brotherhood supporters and there have been more than 1,000 arrests.
The Brotherhood has vowed to continue its demonstrations in the wake of the violence.
Monday's Sinai attack comes as Foreign Secretary William Hague said that the continued unrest across the Middle East represented one of the most significant events of the 21st century so far.

European Commission president Jose Manuel Barroso
He said that events in Egypt, and surrounding countries, could echo for decades and that the international community should expect significant "set backs".
Speaking on BBC's Radio 4 Today programme, Mr Hague said: "What is happening now in the Middle East is the most important event so far of the 21st century, even compared to the financial crisis we have been through and its impact on world affairs.
"I think it will take years, maybe decades, to play out, and through that we have to keep our nerve in clearly supporting democracy, democratic institutions, promoting dialogue and there will be many setbacks in doing that and we should not be surprised when they take place."

Demonstrators inside the al Fath mosque in Cairo
European Union ambassadors are discussing the crisis in Egypt amid international alarm at the growing death toll from unrest across the country.
European Commission president Jose Manuel Barroso and the president of the European Council Herman Van Rompuy on Sunday issued a rare joint statement warning that the EU would "urgently" review its relations with Egypt over the coming days.
Prime Minister David Cameron and French president Francois Hollande called for today's meeting of ambassadors in Brussels to be followed by an emergency session of EU foreign ministers.

A police officer in hospital being treated after violent crashes
Britain has condemned the "disproportionate use of force" by the Egyptian authorities and called on all sides to end the violence and to enter dialogue.
The family of four Irish siblings caught up in Egypt's violence have said their relatives are being held by Egyptian authorities.
Omaima Halawa, 20, her two sisters Fatima, 22, Somaia, 27, and their younger brother Ibrihim, 17, were among hundreds of people cleared out of the al Fath mosque when security forces stormed the building on Saturday.
They were forced to seek sanctuary in the mosque on Friday after violent clashes between supporters of Mr Morsi and the security forces killed more than 80 people.
Speaking from the family home in Dublin, their sister Nasaybi Halawa said that her 17-year-old brother could have been "beaten to hell" in the mosque.

Nasaybi Halawa is worried her siblings have been separated
She said she had been speaking to someone in Egypt who had been inside the mosque who said that men were being beaten.
Miss Halawa said: "The person I'm speaking to told me they were beating men to hell. She doesn't know how my brother looks but she told me 'I can guarantee for you, all the men were beaten there. They didn't exclude anyone from hitting them'."
However, Sky sources understand that the four have been visited in Tora Prison by a Turkish diplomat who reported they were "all well".
Irish diplomats have said they are due to be brought to the prosecutor's office at 2pm on Monday and they were "confident" they would be released.

Security forces arrive at the Cairo mosque
Hundreds of Morsi supporters also fled to the building in the Ramses area of Cairo, shoving furniture against the doors to stop police from breaking their way in.
Speaking from the family home in Firhouse, south Dublin, another sister Nasaybi Halawa said her four siblings were being held at one of Cairo's jails.
She spoke out as it emerged that dozens Muslim Brotherhood supporters had been killed in an incident at an Egyptian prison.
The interior ministry said 36 detainees died after suffocating on tear gas during an attempted prison break.