Rights lawyers Gamal Eid and Mohammed Abdel-Aziz – both members of the Front for the Defense of Egyptian Protesters – said Tuesday that all those detained were in custody by midnight Monday.
They say the number of those still held could be lower since police have been intermittently releasing the detainees.
It's unclear if anyone has been referred to prosecutors, AP reported.
Thousands of police were deployed across much of Cairo Monday to stifle plans for mass demonstrations called to protest the government's decision to surrender the islands of Tiran and Sanafir.