Staff at Portland Prison in Dorset refused to open cells earlier after a spate of attacks by inmates.
The Prison Officers Association (POA) said staff had been attacked "every day this week" with excrement and urine, as well as being punched and kicked.
The POA said the issue, reported in the Dorset Echo, was resolved when managers gave assurances troublesome prisoners would be transferred to another jail.
The Ministry of Justice said the prison was "now running as normal".
A spokeswoman said: "There was a short delay in unlocking prisoners this morning. The regime is now running as normal."
POA vice chairman Mark Fairhurst said: "This morning the staff held a branch meeting and decided it was unsafe to work in those conditions.
"Staff are in riot gear because of the unsafe working conditions."
Mr Fairhurst said glass observation panels in the cell doors were smashed and had not been repaired, allowing prisoners to throw missiles at staff.
Carillion, which provides facilities management services to the prison, said it "continues to work closely with the prison to replace any damaged panels".
Officer 'knocked out'
Another POA spokesman said he understood the incident on Friday morning had been resolved within an hour.
The stand-off comes a week after two prison officers at Portland spoke out about their fear of being attacked at work.
Andy Dubber said he had previously been "knocked out" by a prisoner who had taken drugs, while Tony Walker said he once needed stitches on the back of his head after being attacked.
Last week the Prison Service said it had taken steps to "stabilise the estate by tackling the drugs, drones and phones that undermine security".
HMP/YOI Portland is a male adult and young offenders institution.
An inspection in 2014 found "greater attention" needed to be paid to investigations into self-inflicted deaths and raised concerns over substance misuse.