Egypt disappears dozens of activists in latest state repression
Human Rights Watch said on Sunday that at least 40 rights workers, activists and lawyers have been detained since late October.
These campaigners have been taken to "undisclosed locations", Human Rights Watch said, with families and monitors fearing for their safety due to Cairo's record of running a feared and repressive prison system.
"Many of those arrested were people who provided humanitarian and legal support to families of political detainees," HRW said, demanding the government reveal details of the activists' whereabouts.
The rights group spoke with a lawyer and three activists who had been "in direct touch" with the families of those detained, it said.
"The Egyptian security agencies' repression now extends to disappearing those brave men and women who have been trying to protect the disappeared," said Michael Page, deputy Middle East and North Africa director at Human Rights Watch.
As many as 80 people could have been detained in the latest wave of arrests of human rights campaigners, but HRW said it could only verify 40 names.
"[Egypt should] immediately reveal all the detainees' whereabouts, release all of those arrested solely for exercising their rights, and bring any others swiftly before a judge to review their detention," the group added.
Some of those detained were linked to the Egyptian Coordination for Rights and Freedoms human rights group.
It has come under fierce attack from pro-government media in recent months.
Hoda Abdelmoneim, a 60-year-old lawyer and former spokesperson for the Egypt's Women Revolutionary Coalition - a group allegedly close to the Muslim Brotherhood - was among those detained.
Also arrested was Aisha Khairat al-Shater, the daughter of a former Muslim Brotherhood leader currently in detention, along with her husband, lawyer Mohammed Abu Horayra.
The Muslim Brotherhood - which won Egypt's first democratic elections in 2012, was overthrown by the military a year later.
President Abdel Fattah al-Sisi has outlawed the group and branded it a "terrorist" organisation, with thousands linked to the Muslim Brotherhood detained by the regime.
Egypt's secular opposition and human rights activists have also been caught in the drag net.
Hundreds of Muslim Brotherhood leaders and supporters have been sentenced to death or lengthy jail terms after speedy mass trials, deemed unfair by most observers.
Amnesty International denounced the arrest of at least 19 lawyers and human rights activists, including Shater and Abdelmoneim.