Prominent Egyptian dissident Alaa Abdel Fattah was detained Sunday morning beyond his overnight probation, his family and a judicial official said.
"We don't know where Alaa is... The police station says he's most probably at state security prosecution... we seriously don't have any information about him," his sister Mona Seif, also a well-known activist, said on Twitter early Sunday.
Abdel Fattah, 37, is serving five years of probation after completing a five-year jail sentence in March for staging a protest against a 2013 law effectively banning public gatherings.
His probation requires him to report to a police station near his home in Cairo at 6:00pm every evening, and stay overnight in a cell until 6:00am.
On Sunday morning however he was not released.
Officers denied his mother Laila Soueif access to the police station and refused to say why he was not released.
The activist and computer programmer's whereabouts remain unknown and no word has been given on when he might appear before prosecutors, the judicial official told AFP.
Abdel Fattah told AFP in June that security personnel had ordered him twice to stop talking about his probation publicly or face being sent back to jail indefinitely.
A prolific tweeter, he has commented on small-scale protests in Egypt in the last two weeks.
The demonstrations followed an online call to dissent by an exiled Egyptian businessman.
Some 2,000 people have been arrested in the past week in what may be the largest expansion of an ongoing crackdown in Egypt since 2013, rights groups say.
Authorities have detained other prominent critics including well-known academics, politicians and lawyers.
Abdel Fattah has advocated on behalf of other detainees, calling for a more humane and flexible system of probation.