A major and ongoing disturbance by inmates at HMP Birmingham has seen officers lose control of four wings of the prison.
One inmate is reported to have been seriously injured, with others said to be burning their prison records, cutting through gate chains and even arming themselves with stolen prison officers' batons.
Pictures said to be posted on social media by inmates during the disturbance also show some wearing riot helmets and holding riot shields.
The unrest is believed to have begun just after 9am on Friday (16 December) when a staff member's prison keys were stolen after they were "rushed" by inmates.
Specialist riot officers were called to take control of the the privately-run prison – formerly known as Winson Green – but the disturbance soon spread from two to four wings.
The Victorian category B jail, which is close to the city centre, is designed to hold 1,450 adult male prisoners.
Security company G4S, which runs HMP Birmingham, said all prison officers have been accounted for.
The national chairman of the Prison Officers Association (POA), Mike Rolfe, said the disturbance was "another stark warning to the Ministry of Justice that the service is in crisis".
He said: "The whole facts surrounding the incident at HMP Birmingham are not known. National resources have been deployed to deal with this incident involving approximately 300 prisoners within two prison wings. No staff injuries have been reported and staff are currently monitoring the situation and awaiting national support, through mutual aid."
Alex Cavendish, a former prisoner who campaigns for penal reform, said the unrest is "probably most serious riot in a B category prison since Strangeways went up" in 1990.
He said prisoners had taken control of the prison gym and that they were attempting to break into the pharmacy.
There has not yet been official confirmation of the number of casualties.
The unrest comes after a string of warnings about a lack of safety behind bars in prisons across England and Wales.
Ministry of Justice statistics show riot officers have been called out to HMP Birmingham on five separate occasions between January and August of this year.
The managing director for G4S custodial and detention services, Jerry Petherick, said: "Our teams withdrew following a disturbance and sealed two wings, which include some administrative offices. The disturbance has since spread to two further wings. All staff have been accounted for.
"Additional officers have arrived on site and we have deployed canine units within the prison. West Midlands police helicopter is also in attendance. We are working with colleagues across the service to bring this disturbance to a safe conclusion."