Order in Greece's largest prison was restored by Riot police, while another 10 facilities were in a state of unrest sparked by the alleged beating of an inmate by guards.
A justice ministry spokesman said "order has been fully restored" at the high security Korydallos Prison in Athens, where police fired tear gas and stun grenades after some inmates tried to escape during the unrest. Seven inmates were injured, two of whom were taken to the prison hospital for treatment.
"All the prisoners have now returned to their cells," the ministry spokesman said, speaking on customary condition of anonymity.
Police also intervened to take inmates back to their cells at a prison in Patras, southwestern Greece, and were on standby outside other penitentiaries.
The protests started Monday at Malandrinos Prison in central Greece, where the guards allegedly assaulted a bank robbery suspect who is in pretrial detention there. Inmates at other prisons joined in, demanding better detention conditions at the overcrowded facilities, and that judges show greater clemency in sentencing.
"The government will not negotiate upholding the law ... but we are willing to listen to prisoners' grievances," Justice Minister Anastasios Papaligouras told state-run NET television.
At Malandrinos, which houses 440 mostly violent offenders, dozens of protesting inmates remained on the roof of the building as police arrived from nearby towns and gathered outside the prison.
Justice ministry officials were negotiating with a delegation of inmates, the ministry spokesman said.
Protests also occurred at penitentiaries near the cities of Larissa and Trikala in central Greece, Patras and Nafplio in the south, Diavata near Thessaloniki, Komotini and Corfu in northern Greece, as well as Alikarnassos and Hania on Crete.
Authorities said they were not aware of any injuries.
The justice ministry said it is investigating the beating allegation.