The leader of the April 6 Youth Movement, Amr Ali was sentenced to three years in prison by an Egyptian court on Monday.
Ali, who served as the group's general coordinator, was also fined LE500 after being convicted of calling for protests and the overthrow of the regime. Three other people in the same trial were handed the same sentence, but in absentia.
Prosecutors had charged the defendants with rioting, resisting authorities and joining an illegal group, among several other charges.
Ali was arrested at his home in the governorate of Menufiya, north of Cairo, on September 22, there was no official acknowledgement of his arrest for several days, and the April 6 group had feared that Ali, 32, was the subject of a forced disappearance.
In early October, it emerged that he was in Tora prison in Cairo.
The April 6 movement has denounced Ali's arrest and "demanded his immediate release." His arrest has also been condemned by the liberal-leaning al-Dostour Party and the Strong Egypt Party, headed by former presidential candidate Abdel Moneim Aboul Fotouh.
The April 6 Youth Movement was created on Facebook seven years ago in solidarity with a planned textile workers' strike in the industrial city of Mahallah in the Nile Delta. The strike, which was infiltrated by plainclothes policemen who intimidated the workers, never took place.
The youth group was once credited for being a significant force behind triggering the 18-day uprising that ousted Egypt's 30-year President Hosni Mubarak. Since then, it was subjected to a fierce smear campaign before being banned by court order in 2014. Its first leader, Ahmed Maher, has been jailed for months.
Maher, the first general coordinator of the movement, is serving a three-year prison sentenced, handed to him in December 2013, for violating the protest law.