Egypt’s police have detained a prominent rights lawyer who backed protests against the decision of President Abdel Fattah el-Sisi to transfer the sovereignty of two Red Sea islands to Saudi Arabia.

Malek Adly was arrested late on Thursday by plain-clothed policemen, his lawyer Mahmoud Belal said Friday.

A judiciary official said Adly has been remanded in custody for at least a fortnight on suspicion of "attempting to topple the ruling system" and "spreading false news". His detention is subject to extension.

The official further said the rights lawyer is also accused of joining a "group aimed at annulling the constitution."

The prosecution, however, has not elaborated.

"We asked them 'what is that group, what is it called, or who are its members?' There were no answers to these questions," Belal said.

Earlier this month two journalists were arrested for similar charges which sparked outrage among the reporters who called for a presidential apology for the unprecedented police raid on the Syndicate of Journalists and the sacking of the interior minister.

Egyptian police arrested hundreds of people in the days leading up to and during April 25 mass protests against Sisi’s decision to transfer the sovereignty of Tiran and Sanafir to Saudi Arabia. The protesters also called for "the fall of the regime".

The Egyptian government has been cracking down on opposition since former president, Mohamed Morsi, was ousted in a military coup led by ex-military chief Sisi, the current president, in July 2013.