A disturbance is under way at a privately-run jail with reports of a "badly injured" prisoner and inmates burning and destroying their files.
Security firm G4S, which controls HMP Birmingham, confirmed the incident began at just after 9am and involves two wings of the prison.
The Victorian category B jail, which was built in 1849 and is close to the city centre in Winson Green, can hold 1,450 adult remand and sentenced male prisoners.
A G4S spokesman said: "We are responding to an ongoing incident at HMP Birmingham this morning which began just after 9am involving two wings of the prison.
"We are working with colleagues across the service to bring the incident to a safe conclusion."
Prison affairs academic and blogger Alex Cavendish told the Press Association an "inside informant" told him the trouble started with lights being broken and inmates controlling fire hoses.
"The officers were then, as they are instructed to do, trying to get as many prisoners locked in their cells as possible to contain it," he said.
"While one of the officers was putting a prisoner in the cell he was threatened with what appeared to be a used syringe."
Mr Cavendish said while this officer was distracted by the threat, "another inmate came up behind, snatched the keys from his belt and snapped the security chain".
He told the Press Association that once prisoners have control of the keys, protocol tells the officers to "withdraw to a place of safety" and said they "abandoned the wing" where the incident started.
The 53-year-old said inmates have since gained access to the offender management unit (OMU), where their paper records are stored and which are now being burnt.
"I am hearing there is a very badly injured casualty (prisoner), and the prisoners are throwing computers out of the OMU window - destroying records," Mr Cavendish added.
Describing the incident as "probably most serious riot in a B category prison since Strangeways went up" in 1990, he said: "It is a very serious situation and I think the fact they have now accessed the key offices like the offender management unit - that is where all the offender records are.
"If they destroy all of those records it is going to take months or even years to rebuild the information."
It is understood a control command suite, designed to deal with major incidents at the prison, has been set up in response to the trouble.