Masked policemen raided Hassan Sultan's apartment at dawn last month and whisked his three sons to an unknown location, before charging them with membership of a "terrorist" group.

The siblings are among dozens of youth activists who have been arbitrarily arrested in the past two weeks as authorities press a crackdown on opposition, human rights groups say.

Egypt launched the crackdown, targeting all forms of dissent, after then army chief and now President Abdel Fattah al-Sisi overthrew his Islamist predecessor Mohamed Morsi in 2013.

Morsi's Muslim Brotherhood was declared a "terrorist" organization, and hundreds of people were killed.

More than 40,000 people have been detained, according to Human Rights Watch, with hundreds sentenced to death in speedy mass trials.

On May 26, the policemen wearing uniforms from special forces units stormed the Sultan home in search of his three sons, students of the prestigious Al-Azhar Sunni Muslim seat of learning.

"It happened early in the morning. They blindfolded me and my sons, tied us from behind and made us sit on the floor like prisoners of war," Sultan told AFP.

"We haven't seen them since their arrest, but rights lawyers say they were charged with belonging to a terrorist group and that they are being held in a Cairo police station."

Sultan denies his sons were affiliated with the Brotherhood.

Freedom for the Brave, a group of human rights lawyers who track political detainees and provide them with legal assistance, has documented the disappearance of 163 people since April.

Some of them were eventually traced, mostly in detention, but the fate of 66 is still unknown.

And in the past 10 days alone, 23 activists have gone missing, said, prominent activist Mona Seif.

"Most of them were taken from their homes, streets or from outside their offices by people in civilian clothes," she said, adding that their families are demanding an investigation.

Pictures of the missing have been posted on Facebook and Twitter, as friends and family desperately search for information on their whereabouts.

The Facebook page "Where is Israa al-Tawil?" named after a 23-year-old female university student and freelance photographer who has been missing since June 1 has already attracted 10,000 followers.

Her father, Mahfouz al-Tawil, said Israa had gone out with friends after taking an exam.

"Her phone was switched off. We haven't heard anything about her since then," he said.

Like many parents who have a child missing, Mahfouz blamed the police for his daughter's disappearance.

Several activists from the now banned April 6 group, which spearheaded the 2011 uprising against Morsi's predecessor Hosni Mubarak, have also been jailed in recent days.

They "were campaigning to organize a general strike on June 11 to protest at subsidy cuts that have led to a hike in prices," group member Mohamed Nabil said.

One of them, Ahmed Khattab, "was seized by police on June 1 from a street" and faces charges of membership of "an illegal group and calling for civil disobedience," said human rights lawyer Anas Saeed.

A group of 14 rights watchdogs and political parties have denounced in a joint statement "extra-judicial" measures by authorities, including what they say are "forced and unlawful disappearances."

Other rights groups had earlier warned in a separate statement that "the free hand granted to security authorities ... will continue to drive the country towards more violence."

New York-based Human Rights Watch has also joined the fray, accusing Egyptian authorities of "using unofficial facilities, including military bases and security agency sites to house detainees."

"Torture and ill-treatment at these facilities are routine," HRW said in a report on Monday, outlining what it called "flagrant human rights abuses" during the first year in power of Sisi.

Egypt has rejected these accusations as "baseless and false."

"We don't use these methods. If anyone has proof, they should file a formal complaint to the interior ministry," a senior police official from the ministry's media department told AFP.

He stressed that arrests were only carried out in line with requests from the prosecution.

The state-run National Council for Human Rights has, however, decided to set up a committee to investigate the cases of missing activists, one of its members said.

Police abuses were a key factor behind the 2011 revolt against Mubarak and rights groups now accuse Sisi of installing a regime more repressive than that of the veteran leader.