Police forces arrested Hassan al-Brens, a Muslim Brotherhood leading member in Alexandria, on Thursday in Cairo's Nasr City, sources at the Brotherhood said.
They also arrested Fathi Shehab, a Brotherhood leader and a former parliamentarian.
The police arrested Brens as he was taking part in a march to support former President Mohamed Mursi, the sources told the Middle East News Agency.
The police took Brens to an unknown destination, they added.
Police forces have arrested a number of the Brotherhood's leading figures over the past few days, including the group's General Guide Mohamed Badie, Mourad Ali, Islamist preacher Safwat Hegazi and Ahmed Abu Baraka.