Leading Muslim Brotherhood figure Mohammed al-Beltagi has been arrested, according to Egyptian officials.
Mr Beltagi, secretary-general of the Brotherhood's Freedom and Justice Party (FJP), is accused of inciting violence, state media said.
Former labour minister Khaled al-Azhari was also held.
It comes as Egypt's military moves against Brotherhood leaders following the ousting of former President Mohammed Morsi.
Prosecutors had ordered the arrest of Mr Beltagi on 10 July.
Despite the arrest order, he had appeared on an almost daily basis at the Rabaa al-Adawiyah protest camp in Cairo, which was occupied by supporters of Mr Morsi. The former MP was often seen delivering fiery speeches on the stage.
Hundreds of people were killed earlier this month when security forces stormed the protest camps in the capital. Among them was Mr Beltagi's 17-year-old daughter Asmaa.
Much of the criticism of the Brotherhood in Egypt's media has focused on Mr Beltagi, who is accused of being behind acts of violence allegedly perpetrated by Brotherhood members.
He has continued to call for Egyptians to rise up against the military in recorded video statements while in hiding.
Further pro-Morsi demonstrations are reportedly planned for Friday.
The interior ministry has warned that security forces have the right to use live ammunition against protesters who attack public institutions.
Officials said Mr Beltagy and Mr Azhari, who served under Mr Morsi, were arrested in a flat on the outskirts of Cairo on Thursday.
Egyptian state-run news agency Mena quoted a security official saying it was likely Mr Beltagi would be held at Cairo's Tora prison.
Police have already arrested much of the Brotherhood's senior leadership, including its top figure Mohamed Badie and his deputy Khairat al-Shater. They are on trial on charges of incitement to violence.
Hundreds of other members of the Brotherhood, the Islamist movement from which Mr Morsi comes, have also been detained.
The interim government has said that the Brotherhood is a terrorist organisation.
Police arrested 28 Islamists elsewhere in Egypt on Thursday, the Mena news agency said.