Eight prisoners have spent the night on the roof in a protest at the high-security Maghaberry jail in Northern Ireland.
They took to the roof at about 1900 BST on Friday, and the authorities were still trying to end the protest on Saturday morning.
A Northern Ireland Office spokeswoman said they were a mix of republican, loyalist and non-paramilitary inmates.
It is believed that their protest is about having to share cells.
But sources at the prison said only about four of those in the protest actually shared cells.
All visits to the jail have been suspended for Saturday, the spokeswoman said.
At about the time the protest started, army technical officers were called to the scene to deal with a suspect bomb which turned out to be a hoax.
Investigations into the suspicious object meant a group of 15 prisoners in the
exercise yard were cut off from the main building for a time, but they have since
returned to their cells, the spokeswoman said.
Finlay Spratt, of the Prison Officers Association, said they warned the Northern Ireland Office two weeks ago such a protest could happen if the prison intake increased.
"We, as prison officers who have quite an experience of dealing with prisoners in the Northern Ireland prison Service have been continually saying to management that their policy of doubling these prisoners up is actually going to lead to what is happening.
We have warned them repeatedly but they choose to ignore our advice
Prison Officers Association
"We have warned them repeatedly but they choose to ignore our advice".
About 30 people waving loyalist flags gathered outside the prison but the crowd dispersed before midnight.
Frankie Gallagher of the Ulster Political Research Group, which has links
with the Ulster Defence Association, spoke of growing tensions recently within the
"The conditions inside the prison for the prisoners, regardless of their
backgrounds, are close to inhumane," he claimed.
"It would appear that the prisoners are caught up in difficulties between the
management and staff of the Prison Service and that is impacting on prisoners'
rights, their families and visits."