Security forces arrested Erian, deputy leader of the Brotherhood's Freedom and Justice Party, at dawn in east Cairo where he had been in hiding.
Pictures of Erian circulating on social media, apparently taken during his arrest, showed him smiling and making a gesture symbolising the rejection of the military's ouster of Islamist president Mohamed Morsi in July.
Security forces launched a massive crackdown on Morsi's supporters in August, violently dispersing two protest camps in Cairo and making mass arrests.
Morsi himself has been held incommunicado in military custody since his ouster and is due to go on trial on November 4.
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The arrests have not deterred Morsi's supporters from organising demonstrations, which have deteriorated into deadly street fights pitting them against opponents and security forces.
Morsi's detractors accused him of poor governance and charged the Muslim Brotherhood with trying to monopolise power following the 2011 overthrow of longtime ruler Hosni Mubarak.