A 24-hour tax office strike in a row over privatisation has been well-supported in Northern Ireland, a union official has said.
Several hundred workers took action in Belfast with more protesting across the region.
Members of the Public and Commercial Services (PCS) union across the UK have mounted picket lines in protest at the appointment of two private companies, Sitel and Teleperformance, to run call-handling trials in HMRC tax credit contact centres in Lillyhall in Cumbria and Bathgate in Scotland.
The year-long trials are due to start next month and the union is warning they risk paving the way for privatisation in the department, and come at a time when tens of thousands of civil service jobs are being cut across the UK.
PCS assistant group secretary Karen Taylor said: "Our staff take great pride in the jobs that they do and they are totally against privatisation. They see this deal in the contact centres as privatisation by the back door."
Pickets were held at tax offices in Belfast city centre, although by mid-morning only two people stood outside the building at Great Victoria Street to protest against private sector workers being employed to do public sector jobs.
Ms Taylor added: "I can only see the service deteriorating if it is privatised, we have lost over 25% of our staff."
She said: "In Belfast, in our contact centre it has been very well supported, very low numbers have gone in, five had went in and there are over 300 in that building normally."
An HMRC spokesman said: "HMRC is not privatising existing HMRC contact centre jobs but we are determined to improve the service we provide to our customers.
"This means considering a variety of options including drawing on the knowledge and experience of external contact centre operators. Industrial action is unwarranted and unnecessary."