Two prisoners are staging a rooftop protest at Magilligan Prison in County Londonderry, the Northern Ireland Prison Service has said.
They escaped onto the roof of a single storey building while being moved across the prison yard on Wednesday.
They have spoken with the prison's Free Presbyterian chaplain but the reason for their protest is not yet known.
Shortly after 2100 BST they ripped up tiles and started a small fire, which was put out by prison staff.
Prison visits were suspended, but are exected to resume on Thursday.
One of the prisoners is serving a sentence for armed robbery and the other for robbery.
In a statement the Prison Service said they were negotiating with the men.
"No members of staff or prisoners have been injured and all other inmates are currently secure in their cells," the statement said.
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