The Mada Masr website said Bahgat, 37, was questioned for nearly nine hours and then taken to the prosecutor’s office.
“Hossam called his lawyer and told him he was being accused of having published false information and lies that were damaging to the public interest,” Lina Attalah, the website’s chief editor, told AFP.
Lawyer Khaled Ali who attended the interrogation said Hossam was questioned over a report on military trials on suspicion of “publishing false rumours and false news that harms the armed forces”.
Bahgat was to be detained overnight and the prosecution would decide on Monday whether he will be released or officially charged and face a military trial, the lawyer told AFP.
Human Rights Watch describes Bahgat, founder of the Egyptian Initiative for Personal Rights, as “a leading defender of civil rights and liberties in Egypt” who played a prominent role in the 2011 revolution that toppled long-time president Hosni Mubarak.