Over 2,000 low-paid civil servants have walked out in Northern Ireland in a row over redundancy compensation.
With the economy still struggling and massive public sector budget cuts planned, pickets were launched across the country at the start of two days of action.
Courts, ports, job and tax centres across the UK were hit by the walkout by members of the Public and Commercial Services Union.
A rally was also held in support of the strike at lunchtime in Belfast.
PCSU representative Karen Taylor said there had been 90% involvement of staff.
"They are paying out money hand over fist to the bankers yet they are prepared to rip up the contract rights of their own employees," she said.
The union said civil servants will lose up to a third of their entitlements, worth thousands of pounds.
The Government said other civil service unions agreed the changes were fair.
Ms Taylor, who works at Revenue and Customs, said most staff could lose their entitlement to the old redundancy payments.
"We hope the ministers will come back around the table," she added.
She said the union had come up with alternative proposals to make the savings.
"There doesn't seem to be the political will there to come to a negotiated settlement," she added.
The walkout by members of the union is the biggest outbreak of industrial unrest in the service since 1987, with further disruption planned in the run-up to the general election.
The union is protesting over changes to the Civil Service compensation scheme, which it says will "rob" civil servants of up to a third of their entitlements, worth thousands of pounds, when they leave their jobs.
The union said members of the public showed support to strikers at picket lines across the country on Monday morning.
Officials warned that driving tests would be cancelled, court cases delayed and queues would build up at ports and airports.
Cabinet Office Minister Tessa Jowell said: "The changes to the civil service compensation scheme were agreed with five of the six civil service unions after 18 months of negotiation and consultation. These unions all agree with us that the resulting deal is fair for staff and taxpayers."
© Press Association