Ahmed Maher, the jailed former head of the April 6 Youth Movement, has joined a growing hunger strike movement.
Maher, who founded the now-banned movement, stopped eating on 15 September, April 6 member Zizo Abdou said.
The hunger strike has been gaining momentum with activists, journalists and supporters inside and outside of jail joining to denounce a restrictive protest law and to call for the release of political detainees.
Maher is serving a three-year jail term – along with two other prominent activists, Ahmed Douma and Mohamed Adel – on charges of holding unauthorised demonstrations and using force against the police.
A Cairo court banned all activities of the April 6 Youth Movement, ruling on a lawsuit that accused the group of espionage and defaming the Egyptian state.
The group, which played an important role in the January 2011 uprising that toppled Hosni Mubarak, was subject to a campaign of defamation that accused its members of being agents of foreign countries, paid to stir instability in the country.
The move was criticised by rights advocates, who say the ruling was part of a crackdown on dissent.
Recent statistics published by a Facebook page called "We've had it up to here," which tracks the number of hunger strikers, said that 60 detainees were participating, along with nearly 100 supporters outside of jail.
A number of political parties have also expressed solidarity with the hunger strikers, which continues despite a court's decision on Monday to release on bail activist Alaa Abdel-Fattah and two others after they were imprisoned on charges of violating the protest law.