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.
Egyptian officials have denied carrying out any enforced disappearances, but the association says it has documented more than 1,300 cases.
They include Mr Metwally’s son Amr, who went missing in July 2013 shortly after the Egyptian military overthrew President Mohammed Morsi.
Mr Metwally was arrested on his way to Switzerland for this week’s meeting of the UN Working Group on Enforced or Involuntary Disappearances.
He is now being held at the high-security Torah prison on charges including establishing an illegal group.
A leading rights campaigner said his arrest was part of the government’s strategy to ensure that the only information that emerges from Egypt is the official line.
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).