Prison staff in England and Wales are taking part in widespread protests over “unprecedented violence” in jails, the Prison Officers’ Association has said.
Steve Gillan, POA general secretary, called for union members to protest outside their work from 07:00 BST on Friday until “instructed otherwise”.
Staff have been protesting in London, Liverpool, Manchester and elsewhere.
Prisons Minister Rory Stewart said the POA was “irresponsible” for calling for the action, which he said was illegal.
The POA said protests were taking place outside the “majority” of prisons.
BBC home affairs correspondent Danny Shaw said between 50-60 prison officers were protesting outside HMP Manchester and there were significant protests at HMP Wandsworth and HMP Pentonville, in London, as well as HMP Nottingham.
More than 100 officers were protesting outside the entrance to HMP Berwyn, in Wrexham, while about 50 staff were outside HMP Swansea.
The Ministry of Justice said “contingency plans” had been put in place and that it was seeking an injunction to stop the protests, which could be heard in court later.
Mr Stewart added: “Prison officers do vital and important work and we urge them to return to their duty stations, in line with their obligations to the law and the prison service.
“We are deploying our contingency plans but, by not turning up for work, these prison officers are putting their fellow staff and inmates at risk.”
Mark Fairhurst, chairman of the POA, said members had been instructed to ensure there were enough staff on duty inside prisons to deliver medication to prisoners and deal with any disorder.
The protests come after an inspection report on Thursday found inmates had effectively taken control at HMP Bedford.
Brian Cooper, branch chairman of the POA, said conditions at the Bedfordshire jail were unsafe “every day”.
“This year so far we’ve had an officer who’s had his head stamped on and needed emergency surgery for a bleed on the brain.
“We’ve had an officer have his arm broken with a pool cue when he was attacked by a prisoner.
“Another officer has a fractured eye socket with part of the bone detached and might lose the ability to move his eye properly, permanently.”
The POA accused the government of overseeing “the demise of the prison service over the last eight years”.
In a statement, Mr Gillan said ministers had been “paying lip service” to the safety and human rights of prison staff.
“We will now be demanding that the government provide safe prisons, meet our demands to improve personal protective equipment, reduce levels of violence and overcrowding,” he added.
The union said the decision to call for a protest followed a letter from the chief inspector of prisons, issuing an urgent notification notice at HMP Bedford, published on Thursday.
Inspectors said they found HMP Bedford prisoners regularly ignored rules, that there was a smell of drugs “pervading” some wings and an infestation of rats.
Peter Clarke, the chief inspector of prisons, sent a letter to Justice Secretary David Gauke, which means the government has to publish a response and plan of action for the jail within 28 days.
It is the fourth jail to be subject to the “urgent notification” process after Nottingham, Exeter and Birmingham.