Prison officers across England and Wales have staged unofficial walkouts in protest over plans to change the way they work.
They say they have been "undermined" over proposals to reform the system of offender management within jails.
There is also concern over safety, with almost 5,000 assaults on prison staff last year.
The Prison Service has said it will be "continuing discussions" with unions to address their concerns.
The Prison Officers Association (POA) told its members to gather outside each prison and hold an hour-long branch meeting between 08:00 BST and 09:00 BST.
Prison officers are technically banned from going on strike but in 2012 they took part in walkouts over pension reforms.
The POA says it is angry that its members have not been consulted over the government's plans to change the way prisoners are supervised during their sentences - which it believes could lead to redundancies.
It has accused prison officials of a "complete lack of engagement" over the plans.
In a joint letter, the POA's national chairman, Mike Rolfe, and general secretary, Steve Gillian, say the reforms will lead to a diminishing of workers' rights and add that they make "no apology" for asking members to call the walkout meetings.
They wrote: "The Prison Service has been in perpetual crisis for a long period of time.
"Many good, committed and hardworking colleagues, our members, have long been struggling to deal with the increasing violence, challenging working environments and destructive budget cuts that have seen a drastic reduction in staffing and have made our establishments dangerous and unsafe places to work."
They added: "We must try and protect and challenge from the ill-conceived, under-resourced and dangerous attack on our terms and conditions and working practices."
A POA spokesman said he expected between 5,000 and 6,000 prison officers to have taken part in the walkout, with the numbers showing the "strength of feeling" of its members.
He added that members had been told to be "responsible" and that patrols took place on prison wings while the meetings were taking place.
Prison officer Mark Fairhurst, a member of the POA's national executive committee who attended the meeting at Liverpool Prison, said proposed changes to terms and conditions had been made with no consultation, adding: "This is completely unacceptable. They need to sit around the table and discuss these things."
He said the plans included outsourcing offender management to probation teams, as well as scrapping the rank of supervising officer.
He added: "We're now in a situation where it's a perfect storm - there are rising violence levels and reduced staffing levels. We're not prepared to put up with it any more."
Prison officer Dave Todd, a member of the POA's national executive committee for London and Kent, said there could be redundancies - though it was not yet clear how the reforms would affect staff.
He said members were "not opposed to reform and change" but that no detail had been disclosed.
On Wednesday the BBC reported that prison officers at five prisons in England had staged unofficial walkouts over the past five months to raise concerns about safety.
In May, four members of staff refused to carry out their duties at the Mount prison in Hertfordshire, while 40 prison officers at Holme House jail, in Stockton on Teesside, withdrew in protest about changes to the regime.
Further unofficial action occurred last month, when officers on two wings at Swinfen Hall in Staffordshire left prisoners locked in their cells because of safety concerns.
A Prison Service spokesman said: "This morning's unlawful industrial action lasted an hour or less at most prisons. All staff have now resumed normal working. An appropriate level of security was in place across the prison estate at all times.
"Strike action is unlawful for prison officers. We are in regular contact with unions and are continuing discussions to address any concerns they have."
He added that "safety and security" of prisons was a priority with "well-established plans" in place to respond to action.
Those who took part in the unofficial walkouts lost an hour's pay, the Prison Service said.
Shadow prisons minister Jo Stevens said: "Any sensible employer undertaking major changes in the workplace would actively engage with their workforce right at the start, so those changes have the best chance of success.
"Our prisons are overcrowded, understaffed and violence against prison officers is at record levels. Prison officers rightly feel aggrieved about the lack of consultation with them."
The Ministry of Justice announced almost £13m in funds to improve safety in prisons last month.