The Department for Work & Pensions has downplayed the impact of the biggest civil service strike in 13 years.
Thousands of workers are on strike
It said the vast majority of job centres remained open on Monday, despite the decision by up to 85,000 workers to go on strike.
The Public and Commercial Services (PCS) union said 80-90% of its members had walked out of job centres and benefits offices, disrupting services.
The action was part of a long-running dispute over pay.
The strike was called after a breakdown in pay talks, but the DWP described the action as "completely indefensible".
Strike 'well supported'
The DWP said that nearly 90% of job centres were open and providing a service.
"Early reports suggest that only 146 offices have closed as a result of action," the DWP said in a statement.
"We have plans in place to minimise disruption to customers."
But a PCS union spokesman said the strike action had "massively disrupted" job centres and benefits offices, and many were operating with a "skeleton staff".
The union said that up to 90% of its members had supported the strike.
"This strike has been incredibly well supported, even more than we had expected," said PCS general secretary Mark Serwotka.
"It clearly demonstrates members' anger at the levels of poverty pay and unequal salaries in the civil service."
The driving examiners' dispute - which takes place on Tuesday - will lead to the cancellation of 5,000 driving tests and a backlog in the system, Mr Serwotka said.
"The reason civil servants are doing this is because they are on poverty pay," he told the BBC.
"They provide frontline services yet they are on poverty pay."
Up to 5,000 driving tests could be cancelled
Mr Serwotka said a quarter of civil servants earned less than £13,750, with starting salaries in the Department of Work and Pensions as low as £9,900.
And he argued civil servants were being asked to take a pay cut by accepting a below-inflation pay rise.
He said "sensible negotiations" on the part of the department were all that was needed to move towards an end to the dispute.
After the strike there will be an effective work to rule, with the withdrawal of "goodwill".
Mr Serwotka said this would also cause disruption as "we have people who are constantly working excessive hours to deliver services because there is not enough staff".
Public sector workers have also been angered by reports that senior civil servants are to receive an inflation-busting 9% pay increase which could take some of their salaries near the £300,000 mark.
Outside Stockwell Job Centre Plus, in south London, workers told BBC Radio 4's Today programme they should be paid more.
Jackie Dutton said: "I think it is fair to say the mood inside this office, and other offices like it, is very angry about pay and very angry about appraisal."
Her colleague, John Mahoney, said: "Our feeling about this is that it is low pay throughout the civil service. We hope that managers will be sympathetic to our needs and hopefully they will join us on our picket lines."