Almost 100 workers at the TV Licensing Authority building in Bristol have gone on strike over plans to outsource work from Britain to India.
The company plans to transfer the jobs to India
Some 98 communication Workers Union members began a 48-hour walkout amid fears that hundreds of jobs could go.
Protesters formed a picket line outside the building and handed out leaflets.
A union official said the company had refused to safeguard UK jobs. Capita, which administers the licence, denied jobs would be lost in the UK.
Capita made the decision to outsource work from Bristol to Mumbai in a cash-saving bid.
Union bosses fear the move could lead to large-scale job cuts after the company refused to give guarantees about the long-term future of dozens of posts at the centre.
Disputing the Capita figures, union official David Wilshire said: "We've got 130 people or so here today and the strike is being very well supported.
"It's taking place because Capita have announced the equivalent of 35 jobs will be moved to India and they're refusing to guarantee further jobs will not be moved there in the future."
A spokeswoman for Capita confirmed the equivalent of 35 full-time posts were being created at their Mumbai operations centre over the next six months - but denied UK jobs would be lost.
"There will be no job losses following Capita's decision to increase the amount of back-office work carried out at Capita's site in Mumbai for the TV licensing contract," she said.