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.