The prison guards, who are government workers, started a new strike Thursday to demand salary hikes and bonus payments for late shifts.
They have been walking out sporadically all year, and some prisoners are reportedly unhappy about canceled visits because of the walkouts.
Prisoners rioted earlier this week in Lisbon and held a protest at another prison.
The Justice Ministry noted the new strikes taking place through December and January could cause the cancellation of Christmas and New Year festivities, stoking unrest.
The guards, numbering around 4,000, must by law provide minimum services to ensure prisons remain operational during the strike action.
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