More than 800 Lothian tax workers have gone on strike in a dispute over new working procedures which unions claim "reduce staff to robots".
Hundreds of workers have taken part in the strike
Civil servants at HM Revenue and Customs sites in Livingston and Edinburgh are unhappy with a new "lean" working system.
The Public and Commercial Services Union said work had been "de-skilled".
HMRC said it hoped to resolve the situation quickly and insisted that the system was key to improving services.
The new working practices mean that instead of carrying out a variety of tasks, each member of staff is required to focus on just one area.
Nick McCann, PCS Scottish Organiser, said it had led to a "production line" way of working which was repetitive and stressful.
"We have tried to negotiate with management but have reached an impasse," he said.
HMRC said it was "deeply disappointed" at the strike action.
A spokesman said: "We remain ready to talk to the unions and hope to resolve the situation as quickly as possible."
PCS general secretary Mark Serwotka said: "It is ironic that in an age where the government is placing a huge emphasis on creating a skilled workforce across the economy, that we have one of the biggest government departments introducing working practices designed by consultants that reduce the workforce to robots."
According to the union, the new system had also caused many workers to be referred for repetitive strain injuries.
The one-day strike began at 0700 BST on Thursday and may cause some disruption with the processing of tax returns.
Mr McCann said there had been a healthy turnout to the industrial action.
"According to the number that had turned up so far we will have a turnout of about 90%," he said.
"We balloted about 950 union members so it is very encouraging.
"We are not doing this to inconvenience the taxpayer, but to send a clear and concise message to management that they need to get into discussion about these draconian practices."