Thousands of prison officers are holding a day of protest in a dispute over health and safety concerns, their union has said.
The Prison Officers Association (POA) said as many as 10,000 prison workers across England and Wales will take part in the action, which have been called as negotiations with the government broke down.
The action began at midnight and will last 24 hours.
Steve Gillan, the POA's general secretary, said officers will only provide emergency cover in what is effectively a strike, as prison officers cannot by law take part in one.
A spokesman for the union said: "The continued surge in violence and unprecedented levels of suicide and acts of self harm, coupled with the recent murder and escapes, demonstrate that the service is in meltdown."
The Ministry of Justice has said the industrial action is unjustified and "unlawful".
A spokesman said: "We have been engaged in constructive talks with the POA over the last two weeks and have provided a comprehensive response to a range of health and safety concerns.
"The Government has announced an additional 2,500 frontline officers to help reduce violence in prisons.
"We have well-established contingencies in place to manage prisons and keep the public safe, but we are clear that this constitutes unlawful industrial action, and we will seek remedy in the courts."
Around 60 guards gathered in the car park within the grounds of HMP Pentonville in north London.
Dave Todd, POA representative for London, Surrey, Kent and Sussex, said conditions in prisons were dangerous.
"We need to act to protect ourselves," he said. "It has not come about quickly - it's a build-up over probably years actually. Three years we have been suffering with recruitment retention issues.
"Last month Lewes prison had a riot, Bedford prison had a riot, assaults are at an all-time high. There's mobile phones and drugs in prisons. It's just unsafe.
"To me, prison officers taking this type of action speaks volumes for what's happening inside."
He said that non-union staff were still working inside Pentonville.