Police in riot gear have entered a prison where hundreds of inmates have been involved in disturbances amid a “developing” situation.
Trouble flared at privately-run HMP Birmingham after a prison officer was “rushed” by inmates.
A number of police officers with riot shields have now entered the main gate of the prison.
G4S said the Prison Service had taken over the response due to the scale of the incident.
The Prison Officers Association (POA) said 260 prisoners were initially involved, but this figure was now nearer 600.
The BBC has been 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 about a dozen riot vans and about 50 officers were now outside the prison, with firefighters ready to be deployed.
He 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.
Earlier, specialist Tornado riot teams were dispatched to the category B jail, which houses 1,450 prisoners.
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.”
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.
Jerry Petherick, managing director for G4S custodial and detention services, said West Midlands Police’s helicopter was also in attendance.
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 quashed rumours that offenders had keys giving access to residential areas and that offenders had occupied some blocks and exercise facilities.
“No keys to perimeter access points were taken,” a spokesman said.
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.
I have been speaking to prisoners coming out of the jail who said poor conditions inside the jail have fuelled the build up to the disturbance.
One man waiting for his son earlier told my colleague, the riot started because of a lack of hot water for inmates.
A prison officer was called to the scene and he was threatened with a syringe and his keys stolen which ultimately led to a full scale riot.
All staff have been accounted for but we are yet get any confirmed reports on any injuries.
About 15 people carrying kit bags and wearing G4S boiler suits were seen going into the the prison on Friday afternoon.
All prisoners who are not on the affected wings are believed to be back in their cells, with prison staff awaiting reinforcements before taking control of the other wings.
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.”
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”.