Seven prison officers and one prisoner were taken to hospital after "disorder" at a young offender institution, police have said.
It happened at HM YOI Aylesbury in Buckinghamshire on Friday morning, Thames Valley Police said.
An online prison blog said 30 inmates took part in the "mass brawl".
Police said the injured people had since been released from hospital. The Prison Service said it was investigating the incident with police.
Officers were called to "a report of disorder" at the young offender institution in Bierton Road at about 11:00 BST, but "officer deployment was not required at the incident", a spokeswoman said.
Prison UK blog
"Seven prison officers and one prisoner were taken to hospital for treatment."
The incident came to light in an online blog on Prison UK.
Author Alex Cavendish, a former prisoner, said he was passed the information via "reliable, professional sources inside the system".
"A mass brawl broke out yesterday morning... Prisoners were attacking each other with weapons - and staff," the informant said.
He described the incident as "horrific".
"Staff were trying to save their lives and got battered. Some were seriously injured. Another officer was on the landing unconscious.
"We had ambulances and fire service in, trying to help alongside our healthcare department.
"The wing has been brewing for a while," he added.
A Prison Service Spokesman confirmed that "an incident involving a number of prisoners took place on Friday 21 July".
"We do not tolerate violence against our hard-working staff. Where incidents like this occur, we will always work closely with the police to push for the strongest possible punishment," he added.
The incident is being jointly investigated by the Prison Service and Thames Valley Police.