The spiritual leader of the Muslim Brotherhood, “General Guide” Mohammed Badie, has been arrested in Cairo, Egyptian officials say.
Reports said he was detained at a residential flat in Nasr City.
A state of emergency is in force in Egypt amid turmoil following a crackdown on Islamists in which hundreds have died.
Three days of mourning are being held for 25 police killed in Sinai by suspected Islamist militant extremists.
Almost 900 people, including more than 100 police and soldiers, are reported to have been killed in Egypt since Wednesday, when the army cleared protest camps set up by supporters of deposed President Mohammed Morsi, many of them members of the Muslim Brotherhood movement.
On Sunday, 36 Islamist protesters died in a prison van in the capital, Cairo.
Hundreds of members of the Muslim Brotherhood have been detained over recent days.
He is facing charges of inciting violence and murder over the killing of eight anti-Brotherhood protesters outside the movement’s headquarters in Cairo last June.
Officials and state media said Mr Badie was detained in a flat in Nasr City in north-east Cairo, near the site of one of the protest camps bloodily broken up last week.
Images of Mr Badie shortly after his arrest were shown on the private channel ON TV – a sign that the interim government is bent on humiliating the Brotherhood and weakening it as much as they can prior to any possible negotiations, says BBC regional analyst Youssef Taha.
The arrest comes days after Mr Badie’s 38-year-old son, Ammar, was shot dead during protests in the capital’s Ramses Square.
Mr Badie’s deputy, Khairat al-Shatir, was arrested in the days following Mr Morsi’s overthrow.
Correspondents say his detention will further ratchet up tensions in the country, where an indefinite dusk till dawn curfew is in place, a leaves a power vacuum at the top of the Brotherhood movement.
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