Prison officers were tonight treated for shock and smoke inhalation after disturbances in Maghaberry jail, in County Antrim.
Around 35 loyalists are believed to have barricaded themselves inside Bann House where a fire broke out shortly before 8 p.m. A suspect device was also discovered in the wing.
Fire crews put out the blaze but prison staff had to be treated in the health centre.
A Northern Ireland Office spokeswoman said between 12 and 15 staff were examined but there were no reports of serious injuries.
The disturbances occurred after talks between prison officer representatives, their management and the Government broke up for the evening in London.
Prison staff had threatened a UK-wide strike in support of their Northern Ireland colleagues because of concerns over the safety of employees in the North.
It was unclear tonight what the exact motive for the loyalists' protest was.