A riot involving hundreds of inmates has raged for about 12 hours at a city prison.
Trouble flared at privately-run HMP Birmingham after an officer was "rushed" by inmates.
G4S said the Prison Service took over the incident due to the scale of the disturbance.
All four wings of the prison were secured after a number of officers armed with riot shields entered, the BBC understands.
The Prison Officers Association (POA) said 260 prisoners were initially involved, but the figure climbed to nearer 600.
A prisoner injured earlier in the day was removed, the BBC understands.
There were unconfirmed reports he was stabbed but his injuries are not thought to be life-threatening.
A number of specialist Tornado riot teams were seen entering the category B jail, which houses 1,450 prisoners.
Teams from prisons across the country were also called in to help bring the situation under control.
The BBC was contacted by several men claiming to be prisoners at the jail who said poor conditions were behind the disturbance.
BBC News reporter Ben Godfrey, who is at the scene, said smoke had been seen coming from the jail, but he had been told it was in the prison yard. There were also reports of a second fire within the compound.
Unverified photos purporting to show scenes from the disturbance have been circulated on social media.
In one picture, two men can be seen wearing riot helmets and gesturing towards the camera.
The men, who said they were calling the BBC from inside the jail, cited inadequate staff numbers, poor healthcare and nutrition, and being on "lockdown" in their cells all day as major factors that contributed to the trouble.
Prisoner Andrew Nicholls, who was released on Friday described the condition he experienced while serving his sentence.
"There's no staff, too much Mamba, (a psychoactive drug) and people are dying.
"I've seen someone hang themselves outside my cell. It's terrible - 24-hour lock-up most times. There's two officers for four floors."
G4S denied claims staffing levels were too low.
It said the prison was "run with the required amount of staff" and there had been "no curtailment" in the regime in recent weeks.
Officers initially withdrew from two wings - which included some administrative offices - on Friday morning. G4S confirmed the incident later spread to two further wings.
It is understood the disturbance centres on wings N and P, which hold 250 prisoners, and both have been sealed off. All staff are accounted for.
Alex Cavendish, a former prisoner who is now a prison and crime affairs academic, said trouble flared when lights were broken and inmates took control of fire hoses.
Officers then started locking prisoners in their cells, but one officer was threatened with a used syringe and another prisoner "came up behind him and snatched the keys from his belt".
Staff then "withdrew to a place of safety", abandoning the wing, as they are directed to do in such situations.
He described the incident as "probably the most serious riot in a B category prison since Strangeways went up" in 1990.
He said he was getting reports of one badly injured prisoner, inmates getting into the offender management unit (OMU) and destroying records and the prison gym being invaded.
West Midlands Ambulance Service was called to the site in Winson Green Road at about 12:25 GMT and at least four ambulances were at the scene.
West Midlands Police said "no keys to perimeter access points were taken," despite reports gates had been breached.
From the scene: BBC News reporter Ben Godfrey
It is clear the prison is in the grip of a major riot.
In the last 30 minutes, I have seen six to eight prison vans pull round the side of the building. Could this be the end of the trouble?
There have been a number of vehicles going inside this afternoon and in the last 30 minutes, I have seen six to eight prison vans pull round the side of the building.
One man waiting for his son earlier told my colleague, the riot started because of a lack of hot water for inmates.
A G4S spokeswoman said: "We are responding to an ongoing incident at HMP Birmingham this morning which began just after 09:00 GMT involving two wings of the prison.
"We are in close contact with the National Offender Management Service, and are working with colleagues across the service to bring the incident to a safe conclusion."
The MoJ said specialist teams had been deployed "as part of the ongoing operation to manage the serious incident".
It said the prison's perimeter was secure and there was no risk to the public as the situation "remains contained".
"Significant resources are at the scene and we continue to work with West Midlands Police and other emergency services to resolve the situation," it added.
A spokesman for the Prison Officers Association (POA) said no staff injuries had been reported.
He said the incident "is another stark warning to the Ministry of Justice [MoJ] that the service is in crisis".