Hundreds of prison officers have failed to turn up for work at Northern Ireland's three main prisons.
Maghaberry prison is NI's high security jail
Visits to Maghaberry jail in County Antrim, Magilligan prison in County Londonderry, and Hydebank Young Offenders centre on the outskirts of Belfast, were cancelled on Friday as police officers were called in to provide cover.
The normal prison regime has been suspended at all three jails and prisoners have been locked in their cells.
The Prison Officers' Association has been involved in a long-running row with management over security arrangements for staff at their homes.
The union dismissed a recent meeting with the Prisons Minister Jane Kennedy as "a waste of time".
The Prison Service has condemned the action as "reprehensible".
Director General of the Prison Service, Peter Russell said the action, which he said was unofficial, would do nothing to resolve the issue.
"Staff were warned this morning of the consequences of leaving their posts. It is a breach of their terms and conditions of service and as such they will not be entitled to pay," he said.
"This action increases the potential for disruption in the prisons when there is already a volatile atmosphere."
Finlay Spratt, chairman of the Prison Officers Association, said he supported his members but added that he had not known of the action in advance.
"I fully support any decision they have made.
"If they are stressed out and sick, they have my utmost sympathy. This was to
show government and management that they had had enough."
Recently, dozens of cells were wrecked during trouble at the high security Maghaberry jail.
In September, a review of safety at Maghaberry recommended separating republican and loyalist prisoners.
The move was being introduced in the wake of violent clashes between rival groups in the jail and in the face of a "dirty protest" by a group of dissident republican prisoners.
As well as paramilitary prisoners, Maghaberry houses male and female prisoners, whether they are convicted or on remand, and a number of asylum seekers.