Egypt's military prosecutors on Sunday ordered activist Alaa Abdel Fatah to be jailed for a further 15 days, on charges of inciting violence against the army, according to the semi-official newspaper Al Ahram online.
The extension was condemned by dozens of activists who gathered in Cairo to demand Abdel Fatah's release, dpa reported.
On October 30, military prosecutors sent Abdel Fatah to jail for questioning on charges of inciting attacks on military troops and stealing army weapons during largely Christian protests in central Cairo on October 9.
He has reportedly refused to answer questions from military prosecutors because of his opposition to civilians being tried before military courts - whose rulings cannot be appealed.
Abdel Fatah's mother is on hunger strike in protest against his detention. Laila Sueif, a science professor, says her son's jailing is punishment for his opinions critical of the military rulers.
On November 4, human rights group Amnesty International demanded Egypt's military rulers "immediately and unconditionally" release Abdel Fatah, calling him a prisoner of conscience.
About 12,000 civilians have been prosecuted by the military since former president Hosny Mubarak was toppled by a popular uprising in February, according to Egyptian rights advocates.