A union claims about 10,000 council workers face losing their jobs unless they accept worse pay and conditions under new contracts.
The GMB accused Labour-run Rhondda Cynon Taff (RCT) council of threatening staff with a "lock-out".
It said the contract changes included pay, sick pay, and car allowances.
RCT, which voiced disappointment at the union reaction, said it was reviewing staff contracts because of cuts, and hoped to avoid compulsory redundancies.
The GMB union walked out of talks with the authority over the proposals and has advised members not to sign anything.
They want the lock-out - job loss unless workers sign the new contracts - to be lifted before talks can continue.
Rhondda Cynon Taf has the second highest population in Wales, and GMB officer Gareth Morgan called the actions of the authority "totally unacceptable".
"It's like holding a loaded pistol to people's heads to force them to accept detrimental change or be out of a job with no compensation," he said.
"GMB is well aware of the financial deficit facing the authority and are willing to consult meaningfully on ways of reducing the budgetary deficit of £20m each year for the next three years.
"This sort of gunboat diplomacy has no place in industrial relations particularly when dealing with a Labour controlled authority which is extremely disappointing to say the least."
An RCT spokesperson said the review of staff contracts was due to public sector cuts.
The UK government last week announced spending cuts which will affect the amount of money given to local authorities from central government funds.
An RCT spokesperson said: "As a result of pending cuts to public sector funding, the council is likely to face a funding shortfall of around £60m over the next three years.
"It is envisaged that by reviewing the terms and conditions of employees the council will be able to avoid the need for compulsory redundancies and, in turn, protect hundreds of local jobs within the council.
"We hope we can continue to work with the GMB and the other trade unions during this difficult time to ensure we protect local jobs and important public services.
"The council is disappointed to receive such a reaction from GMB at such an early and sensitive stage in proceedings.
"We hope we can continue to work with the GMB and the other trade unions during this difficult time to ensure we protect local jobs and important public services."
Plaid Cymru South Wales Central AM Leanne Wood called it "outrageous right-wing behaviour".
"The Tories have taken similar action in England but not even they have dared to do the same in Wales," she said.
"The Plaid group on the council have battled to prevent this non-negotiable downgrade of workers' terms and conditions, but the ruling Labour group ensured changes were voted through."
The Welsh Local Government Association, which represents 22 councils in Wales, said local authorities were facing "extremely challenging cuts" to funding and were looking for new ways to cut costs and produce balanced budgets, which they had to do by law.
"Workforce costs make up over 50% of council spending, so terms and conditions cannot be exempt if we are to maintain services and jobs, which will be a priority for the council tax payer and most employees," said a spokesman.