Egyptian investigative journalist and rights activist Hossam Bahgat has been arrested, a website he works for says.
Mada Masr said Mr Bahgat, 37, was questioned by military intelligence for nine hours and could be charged with publishing false information.
Amnesty International said the arrest was “yet another nail in the coffin” of freedom of expression in Egypt.
Tens of thousands are thought to have been jailed since the army overthrew President Mohammed Morsi in 2013.
The Mada Masr report said Mr Bahgat had received a summons from military intelligence on Thursday, and attended its headquarters on Sunday morning.
He was not allowed to take his phone or be accompanied by a lawyer to the interrogation, it added.
Mr Bahgat told the website he could be charged with “publishing inaccurate and false information that harms national interests”. No charges have yet been brought.
He is currently being held at the military prosecutor’s office, Mada Masr said.
The Amnesty statement said the arrest showed the Egyptian authorities were continuing their “ferocious onslaught against independent journalism and civil society”.
“The arrest of Hossam Bahgat today is yet another nail in the coffin for freedom of expression in Egypt,” it quoted Middle East and North Africa Programme Director Philip Luther as saying.
“He is being detained and questioned by the military prosecutor for peacefully exercising his right to freedom of expression and must be immediately and unconditionally released.
“Any charges brought against him must be dropped.”
Hossam Bahgat is the founder of human rights group Egyptian Initiative for Personal Rights.
Since 2014 he has published a series of investigative reports on Mada Masr, including the conviction in August of a group of military officers for plotting a coup with the banned Muslim Brotherhood.