A mass meeting was held at the Ford plant in Southampton at 1000 GMT
Car giant Ford is to cut up to 850 jobs at its UK operations by May.
Between 400 and 500 of the posts are to go at the Transit plant in Southampton. The firm said it hoped the losses would be met through voluntary redundancies.
Bosses will also renegotiate a recent pay deal struck at the south coast site and have postponed a decision on the future Transit production at the plant.
Union Unite called it a "betrayal" and warned cutting pay to meet redundancy payments could lead to strike action.
Tony Woodley, Unite's joint general secretary, said: "These proposals are completely unacceptable and a betrayal of Ford's loyal UK workforce.
"Ford are asking the workers to take a cut in pay to preserve jobs, but workers are asking themselves if their pay is being cut to pay for friends and colleagues to be thrown on to the dole."
The union warned that unless Ford reconsidered the plans, it will have "no option" but to ballot members on industrial action.
The GMB union accused Ford of "going back on its agreement" over a 5.25% pay increase agreed late last year.
The layoffs represent nearly 7% of Ford's UK workforce of 12,900 - with almost half of the Southampton factory's staff of 1,100 affected.
The other 350 job losses will be spread across Ford's remaining UK sites in Basildon, Brentwood and Dunton, in Essex, Daventry in Northamptonshire, Halewood on Merseyside and Bridgend in south Wales.
Gary Alexander, a worker at the factory for 20 years, said: "It's doom and gloom here at the moment and I think this factory's days are numbered.
"They [Ford] went back on their word about the pay offer and they said there would be no more job losses.
"They said the redundancies will be voluntary but they will not get that many."
He ruled out a possible strike saying "nobody wants that".
Justin Bowden, GMB union officer, said: "Ford of Europe made a profit in excess of £1bn in 2008.
"The pay offer reflects last year's rate of inflation and the massive contribution to that profit by the UK Ford workers.
"Yet again Ford is going back on its agreement with its workforce and this time it is on pay."
Another worker at the Southampton factory said a mass meeting had been planned for 1000 GMT.
Ford blamed a fall in commercial van sales for the job losses
"I have only worked here two years so I hope it's not last in, first out," he added.
Sandra Gidley, Lib Dem MP for Romsey, claimed the government had been "slow to act".
She added: "This is tragic news for the plant, the workers and the city.
"That is bad enough, but Ford are creating extra uncertainty by renegotiating the recent pay deal.
"I have been consistently raising concerns for some months, but the government have been slow to act."
She said she was seeking a meeting with business secretary Peter Mandelson over the situation.
Ford has blamed the "unprecedented" economic conditions for its decision.
John Fleming, Ford of Europe chairman and chief executive, said: "As demand across the industry continues to fall, we are facing some immediate and major challenges.
"Those companies which act quickly in taking the right decisions will be those who not only survive but who emerge strongest from this deep recession."
The announcement comes as the Society of Motor Manufacturers and Traders announced that new car registrations fell by 30% in January.