Prosecutors released April 6 Youth Movement founder Ahmed Maher on Sunday afternoon, a judicial source has told Ahram Online.
Maher's arrest was ordered after April 6 took part in demonstrations outside parliament on Tuesday in defiance of a new protest law.
He was charged with rioting, assaulting police officers, blocking roads and violating the protest law.
Maher turned himself in to prosecutors on Saturday.
April 6 is renowned for its role during the popular revolt that ousted autocrat Hosni Mubarak in 2011.
The Egyptian protest law, which bans demonstrations without prior police approval, was passed a week ago by the interim government and has provoked an outcry among rights groups.