Militants killed 25 policemen on Monday in the deadliest attack of its kind in years, as Egypt's army-installed rulers escalated a campaign to crush ousted president Mohamed Morsi's Muslim Brotherhood.
The assailants fired rocket-propelled grenades at two buses carrying police in the Sinai Peninsula, security sources said, just hours after 37 Brotherhood prisoners died in police custody.
The bloodshed came after Egypt's military chief vowed a "forceful" response to violence roiling the Arab world's most populous nation.
Judicial sources said new accusations of inciting the deaths of protesters had been levelled against Morsi, who has been held in a secret location since the military deposed him on July 3.
Former president Hosni Mubarak meanwhile won conditional release in the third of four cases against him, but remained in detention on the fourth, judicial sources added.
The Sinai attack raised fears of a return to the wave of deadly Islamist violence that swept the country in the 1990s.
Egypt is struggling to put a lid on a deep political crisis and violence that has killed almost 900 people in days of clashes between Islamist protesters and security forces across the country.
Western states have condemned the violence and are threatening to cut off billions of dollars in aid, though Saudi Arabia said Monday that Arab nations would step in to fill any gap.
Morsi loyalists vowed new demonstrations on Monday, although a day earlier they had cancelled some marches citing security concerns.
The interior ministry said 25 policeman were killed and two injured in the Sinai attack, which it blamed on "armed terrorist groups".
A border official said afterwards that the Rafah crossing with the Palestinian Gaza Strip, near where the attack occurred, would be closed.
Security sources said another policemen was killed in the northern city of El-Arish, bringing to at least 75 the number of security force members killed in the Sinai since the army toppled Morsi.
The security situation in the Sinai has deteriorated sharply in the past weeks, with near daily attacks by militants targeting police and military installations.
Elsewhere bloodshed sparked by the August 14 security force crackdown on pro-Morsi protest camps in Cairo showed little sign of abating.
Authorities said 37 Islamist detainees died after police fired tear gas in a bid to free an officer taken hostage by prisoners, as the inmates were being transferred to a north Cairo jail.
But the Brotherhood, the once-banned movement from which Morsi hailed, held the police accountable, accusing them of "murder".
They said the incident affirmed "the intentional violence aimed at opponents of the coup, and the cold-blooded killing of which they are targets".
Only hours before the deaths, military chief Abdel Fattah al-Sisi pledged a "forceful" response by security forces to any violence from protesters.
"We will never be silent in the face of the destruction of the country," said Sisi, who overthrew Morsi after mass protests against the Islamist's single year of turbulent rule.
Egypt's foreign minister Nabil Fahmy said Monday in Sudan that his country was on the "right path".
According to an AFP tally, more than 1,000 people have been killed since the mass demonstrations calling for Morsi to resign erupted at the end of June.
The international community has fiercely condemned the violence, with rights group Amnesty International decrying it as "utter carnage".
The group's secretary general Salil Shetty warned the country's government had "stained its human rights record".
In response to the violence, EU ambassadors were recalled from their summer break for a meeting in Brussels, with foreign ministers due to review the bloc's ties with Egypt at an emergency meeting on Wednesday.
The European Union has pledged nearly five billion euros ($6.7 billion) in aid to Egypt but has cautioned this was under "constant review" after Morsi's ouster.
The United States has cancelled joint military exercises with Egypt but stopped short of suspending $1.3 billion in annual aid.
On Monday, US Defence Secretary Chuck Hagel urged Egypt's government to take an "inclusive approach to reconciliation".
The international response has not been uniformly critical, however. Both Saudi Arabia and Jordan have said they back Egypt in its fight against "terrorism".
In Israel, meanwhile, an official urged the West to support Egypt's military.
"The name of the game right now is not democracy," he told the Jerusalem Post.
"The name of the game is that there needs to be a functioning state. After you put Egypt back on track, then (you can) talk about restarting the democratic process there."