Guards blocked entrances at as many as 18 French prisons after two colleagues were stabbed by a radicalised inmate.
In a statement, the main CGT union said attacks against guards are becoming more common and intolerable, and that the penal population was becoming "increasingly vindictive, aggressive and violent".
Earlier today, a total of 18 prisons had been blocked. That had reduced to 10 by mid-morning.
The protest comes after guards were stabbed with a ceramic knife by a prisoner, whose wife was visiting. He locked himself in the room afterwards but no hostages were taken.
The inmate and his wife were subdued in a police raid. The Le Monde newspaper reported that the wife died of her wounds.
French officials described the stabbing as a "terrorist attack".