Five prison officers have been injured in an attack by an inmate.
The attack happened at HMP Bristol on Friday at 14:00 GMT and all five officers "received hospital treatment", a Prison Service spokeswoman said.
Sarah Rigby, from the Prison Officers Association (POA), said the prisoner had been transferred from a high-security jail.
The Prison Service said it was investing in staff safety by issuing bodycams and pepper spray.
Ms Rigby said the inmate "should never have been moved out of the high-security establishment".
'Dangerous hostile environment'
She said those injured had left hospital and were recovering at home and added: "Staff at HMP Bristol have not yet been issued with PAVA [pepper spray] and rigid bar handcuffs and it is possible that the PAVA particularly could have protected staff better during this incident.
"The POA again wishes to reiterate that the employer must issue this protective equipment at the earliest opportunity to offer better protections for the staff carrying out their duties in a dangerous, hostile environment."
An Avon and Somerset Police spokeswoman said they were investigating an assault.
She said the prison officers were "taken to hospital for treatment to injuries which are not believed to be life-threatening" and added: "A suspect was detained by prison staff and is due to be questioned by police officers."
HMP Bristol has about 520 adult male prisoners and a limited number of young offenders, both convicted and on remand from local courts.
It is a category B prison where the majority of prisoners stay for fewer than 12 months.