An Egyptian coalition of rights groups says police arrested at least 237 people during protests in Cairo the previous day against a government decision to hand over to Saudi Arabia control of two strategic Red Sea islands.

Rights lawyers Gamal Eid and Mohammed Abdel-Aziz — both members of the Front for the Defense of Egyptian Protesters — said on 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.

Thousands of police were deployed across much of Cairo on Monday to stifle plans for mass demonstrations called to protest the government’s decision to surrender the islands of Tiran and Sanafir.