At least 24 Egyptian policemen have been killed in an attack by suspected militants in the Sinai peninsula.
The attack on the police convoy, close to the town of Rafah on the Gaza border, was one of the deadliest on security forces in several years.
Interim President Adly Mansour declared three days of national mourning for the attacks, state TV said.
A state of emergency is in force amid wider turmoil following a crackdown on Islamists in which hundreds have died.
Thirty-six protesters died in a prison van in the capital Cairo on Sunday.
Meanwhile, Egyptian prosecutors have ordered the ousted President Mohammed Morsi be detained for a further 15 days while they investigate fresh allegations against him.
He has reportedly been accused of complicity in acts of violence against protesters outside the presidential palace last December.
And separately, a lawyer for Hosni Mubarak has said he hopes the former leader could be released from prison within the next two days.
Lawyer Fareed al-Dib told the BBC that Mubarak had been cleared of one corruption charge and they were waiting for the court to check whether he still had to be held in custody on other counts.
Mubarak is facing a retrial for corruption and complicity in the deaths of protesters during the 2011 uprising which ended in his removal from power.
While it is still no more than a rumour, his release would be seen by many Egyptians as a signal that the military is rolling back the changes that flowed from the uprising of 2011, the BBC's Kevin Connolly reports from Cairo.
There has been widespread unrest since the military deposed Mr Morsi as president on 3 July, following mass protests over his rule since his election a year ago.
More than 830 people, including 102 police and soldiers, are reported to have been killed since Wednesday, when the army cleared protest camps set up by Morsi supporters, many of them members of the Muslim Brotherhood movement.
The European Union announced that its foreign ministers will meet on Wednesday to decide whether to cut some of the billions of euros in aid pledged to Egypt.
In the US, the Obama administration has already cancelled joint military exercises planned for next month and is under pressure to cut off all aid.
EU special envoy Bernardino Leon said the ministers would consider a variety of options - including an arms embargo - but would work from the premise that a political solution to the crisis in Egypt is possible.
In response, Saudi Arabia issued a statement saying it "will always stand" with Egypt and its interim leaders.
Saudi Foreign Minister Prince Saud al-Faisal warned that if Western nations cut their aid packages, "the Arab and Muslim nations are wealthy with their people and resources and will not shy away from offering a helping hand to Egypt".
The off-duty police officers were in two minibuses when they were ambushed in northern Sinai.
They were reportedly ordered to leave the buses before being shot in the back of the head. Their bodies were brought back to Cairo on Monday evening.
In a separate incident, another police officer was killed in the north Sinai town of el-Arish.
In response to the attacks, the Rafah border post into Gaza was closed and security increased at checkpoints on the peninsula.
Attacks by Islamist militants on the Egyptian security forces have surged in northern Sinai since 2011 - they have been close to daily in recent weeks.
As a result the Egyptian military recently intensified a crackdown against militants in the region.
Egyptian deployments in the peninsula are subject to the 1979 peace treaty between Israel and Egypt.
Prison van deaths
Meanwhile, in Cairo on Sunday night, 36 Islamists died as they were being transported between prisons.
Government and military officials said they had been attempting to escape and were suffocated by tear gas in the back of a prison van.
But there were other reports of gunfire.
The Brotherhood accused the interior ministry of murder, saying the killings "show the violations and abuses that political detainees who oppose the July 3 coup get subjected to".
The US state department said it was "deeply troubled" by the deaths and made clear the Muslim Brotherhood should not be banned, spokeswoman Jen Psaki told reporters.
The possibility of a ban has been floated by the interim PM and discussed in cabinet.
Egypt's foreign minister, Nabil Fahmy, speaking on a trip to Sudan, said he believed the country was "on the right path", despite the current crisis.