Soliman, who founded the Center for Egyptian Women's Legal Assistance, was arrested at home and taken for questioning.

"Police came to our house with an arrest warrant for my mother Azza Soliman. We are on our way to Masr al-Gedida police station," Soliman's son Nadeem Abdel Gawad posted on Facebook.

Abdel Gawad later said that she was questioned by a judge while accompanied by a group of human rights lawyers.

The reason for her arrest was not immediately clear but activists said it may be related to an ongoing probe into civil society groups that has sparked alarm from the United Nations.

The move came months after an Egyptian court froze the assets of five other human rights defenders and three non-governmental organisations for allegedly receiving illicit foreign funds.

"This is a new development," said lawyer Gamal Eid, one of the activists whose assets were frozen.

"I think they are escalating, step by step," he said, referring to Soliman's arrest.

Soliman was banned from travel last month. She also learned that her bank account had been frozen and that she was included in the investigation into the civil society groups, Eid said.

The United Nations said it was "extremely concerned" by the September decision to freeze the assets of the activists and NGOs, which stoked fears of an intensified crackdown on civil society.

Amnesty International called the arrest a "worrying escalation".

"Soliman's arrest is the latest chilling example of the Egyptian authorities' systematic persecution of independent human rights defenders. We believe she has been arrested for her legitimate human rights work and must be released immediately and unconditionally. The intimidation and harassment of human rights activists has to stop," said Najia Bounaim, Deputy Director for Campaigns at Amnesty International's Tunis Regional office.