Loyalists today attempted to seize control of part of the high security Maghaberry Prison in Co Antrim.
They confronted staff and demanded keys to one of the blocks. It happened as 12 prisoners, all loyalists, were being transferred to Magilligan Prison in Co Derry.
They were being moved as part of preparations to separate loyalists and republicans at Maghaberry, near Lisburn, following a series of protests by both sides.
Nobody was hurt during today's incident which was described by the authorities as "potentially serious". It lasted for about 40 minutes. Staff refused to hand over the keys.
The decision to separate the prisoners followed a British government review into conditions headed by a former senior Northern Ireland civil servant Mr John Steele.
Additional security measures are being introduced which required some of the loyalists to be moved temporarily from Bann House to Magilligan.
A prison service spokesman said: "Because of the quick thinking and professionalism of staff the situation was quickly brought under control and prisoners were locked up. There were no injuries and no significant damage to property."
He said it was always anticipated that implementing recommendations for separation would be problematic and that paramilitaries would seek to impose their control.
The spokesman added: "The prison service will continue to exercise control and will not be dictated to by prisoners."
PA