OFFICERS at Dartmoor Prison staged a walkout on Tuesday morning, under the direction of union officials.
More than 90 per cent of officers took part in the protest over an imposed 2.7 per cent pay rise, which lasted for one hour.
Vice-chairman of Dartmoor Prison Officers' Association John Masterton said: 'I was absolutely astounded, this shows you how angry they are. Normally you may squeeze somewhere just over 50 per cent turnout, but to get this amount was fantastic.'
Mr Masterton said he was called at 6am on Tuesday, when union officials in London issued instructions to take action.
A meeting was called at 8am, attended by 98 staff members and the walkout began at 9am.
Mr Masterton stressed the action did not affect the prisoners in any way.
'At no time was inmates' safety ever in any jeopardy,' he said.
A prison spokeswoman said: 'The branch called an unauthorised meeting outside the prison this morning at 8am, which the governor only became aware of when he came on duty at 7.40 am.'
She added: 'We have members of staff who are not POA members who carried on with the usual procedures that were required to be done.'
Dartmoor Prison officers defied a Home Office injunction in November last year when they walked out in protest over the pay offer, which they described as 'derisory'.