Egypt's government on Tuesday pressed its relentless campaign against the Muslim Brotherhood of ousted president Mohamed Morsi, dealing a major new blow to the Islamist group by arresting its chief.
Mohammed Badie's detention comes as the Brotherhood reels from the roundup by the security forces of dozens of top members and the deaths of many more in recent days.
Despite its disarray, the Brotherhood moved quickly to name Mahmoud Ezzat, a hawkish deputy in the organisation, as interim supreme guide.
Badie's detention raises fears of new violence in Egypt, where nearly 900 people have died in days of clashes between security forces and Islamist supporters of Morsi.
In the latest bloodshed, militants killed 25 policemen in the restive Sinai Peninsula on Monday , just hours after 37 Muslim Brotherhood prisoners died in police custody.
Judicial sources meanwhile said fresh accusations had been levelled against Morsi, who has been detained at a secret location since his July 3 ouster by the army.
And former president Hosni Mubarak won conditional release in the third of four cases against him, but remained in detention on the last case.
The interior ministry said police picked up Brotherhood chief Badie near Rabaa al-Adawiya square, where more than 280 Morsi supporters were killed on Wednesday as police cleared their protest camp.
It released a video of the 70-year-old, sitting impassively on a sofa, bottles of juice and water placed conspicuously in front of him.
The Brotherhood's political party said the group had appointed deputy Mahmoud Ezzat to assume the role of supreme guide.
Ezzat has been jailed multiple times, and is often referred to as the organisation's "iron man", experts on the group say.
Compared to Badie, Ezzat is a "hawk," said Karim Bitar, research director at the Institute for International and Strategic Relations.
"It could be a signal, showing we can respond to authoritarianism with more authoritarianism," he said.
"In any case, it's a sign that this is not the time of the moderates in Egypt," he added.
Violence has rocked Egypt for days, polarising the country and drawing international opprobrium.
On Monday morning, militants killed 25 riot police in two buses in the Sinai peninsula, in the deadliest such attack in decades.
The interior ministry blamed the attack on "armed terrorist groups" and Egypt closed its border with the Palestinian Gaza strip, near where the attack occurred.
Security sources said another policeman was killed in north Sinai, bringing the number of security force members killed in Sinai since Morsi's ouster to 75.
On Monday evening, coffins draped with Egyptian carrying the bodies of the 25 police arrived in Cairo.
State television offered live coverage of the arrival, and added a black mourning strip to the "Egypt fighting terrorism" banner it has run for days.
The Sinai attack came hours after 37 Muslim Brotherhood detainees died as they were being transferred to a north Cairo jail.
Authorities said they suffocated on tear gas fired by police trying to free an officer the prisoners had taken hostage.
But the Morsi's Brotherhood accused the police 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".
United Nations chief Ban Ki-moon said he was "deeply disturbed" by the deaths and called for a "full investigation to ascertain the facts surrounding this incident."
And rights group Amnesty International on Monday decried violence in the country as "utter carnage".
The group's secretary general Salil Shetty warned the country's government had "stained its human rights record".
Human Rights Watch called on Egypt's rulers to "urgently reverse" instructions for police to use live ammunition against protesters.
The UN rights office said it was pressing Egyptian authorities to let it deploy monitors in the crisis-wracked country.
Spokeswoman Liz Throssell told reporters in Geneva that the office was seeking a green light to send "several" human rights observers to assess the situation.
Last Thursday, UN rights chief Navi Pillay demanded an "independent, impartial and credible" probe into the bloody crackdown by Egypt's security forces, saying anyone found guilty of wrongdoing should be held to account.