Human rights groups have called on the Egyptian authorities to release a prominent lawyer who investigates alleged enforced disappearances.
Ibrahim Metwally Hegazy was arrested at Cairo International Airport on Sunday as he prepared to fly to Geneva to address a UN meeting.
He had been due to speak about cases including that of Giulio Regeni, the Italian murdered in Egypt last year.
Egypt has denied accusations that the student died in custody.
However, officials have admitted that security services were monitoring Mr Regeni, a University of Cambridge PhD student who was researching trade unions.
Mr Metwally is a co-founder of the Association of the Families of the Disappeared, a network of individuals whose loved ones have been subjected to enforced or involuntary disappearances.
Among the cases it has documented is that of Mr Metwally's son, Amr, who went missing in 2013 shortly after the military overthrew President Mohammed Morsi.
Orla Guerin, BBC News, Cairo
For years, Ibrahim Metwally has championed the cause of the disappeared in Egypt. Suddenly on Sunday, the lawyer joined their ranks.
Mr Metwally vanished as he was about to fly to Switzerland to address the UN Working Group on Enforced or Involuntary Disappearances. He is now being held in Cairo's high-security Torah prison on charges including establishing an illegal group.
He founded the Association of the Families of the Disappeared, which has recorded 1,300 cases in the past two years. Most people reappeared in custody after a few weeks, facing terrorism charges.
A leading human rights campaigner told the BBC that Mr Metwally's arrest was part of the government's attempt to ensure the only information that emerges from Egypt is the official line.
"The government's strategy at the moment is to lock down Egypt, and isolate it from the rest of the world, ensuring only the voice of the government comes out," Mohamed Lotfy said.
Mr Metwally's family was not initially informed of his arrest and only discovered his whereabouts on Tuesday evening when his home was raided, according to the Egyptian Commission for Rights and Freedoms (ECRF).
The Supreme State Security Prosecution later ordered that Mr Metwally be detained for 15 days at Cairo's Torah prison pending further investigations.
The ECRF said he was suspected of managing a group created against the law, spreading false news, and co-operation with foreign organisations.
The other co-founder of the Association of the Families of the Disappeared, Hanan Badr el-Din Othman, has been subjected to "preventative detention" since May, when she visited a prison while seeking information about her missing husband.
That month, President Abdul Fattah al-Sisi ratified a new law regulating non-governmental organisations. The move was condemned by human rights groups.
The law states that NGOs are prohibited from conducting activities that "harm national security, public order, public morality or public health", strictly controls their funding, and gives the government the authority to monitor their activities.
Violations are punishable with sentences of one to five years in prison, and a fine of 50,000 to 1,000,000 Egyptian pounds ($2,830 to $56,650).