Thousands of jobcentre and benefit office workers began a 48-hour strike today, closing offices across the country.
Members of the Public and Commercial Services (PCS) union are striking over a dispute about pay rates, which start at £10,300 a year, and "unfair" bonus schemes.
The union claimed the strike was "solidly supported", despite Department for Work and Pensions estimates that only 41,000 staff are taking part - less than half of the union's estimate of 90,000.
The PCS said support had exceeded that in the previous two strikes and that even more people would be on strike tomorrow.
Union membership in the Department for Work and Pensions had increased by 14,000 since the start of the dispute, it added.
Only 15 staff out of 120 went to work today in an employment office in Grimsby, and only 14 out of 268 staff at a social security office in Glasgow were at work. But in Pontefract, West Yorkshire, turnout for the strike was not as good as for previous strikes.
PCS general secretary Mark Serwotka said: "The turnout today has been excellent with staff staying away from work in protest against a divisive and unfair bonus system and the continuing problem of low pay.
"It's high time management recognised that their highhanded tactics aren't working and started to address the concerns of the thousands of hardworking staff in the department."
Mr Serwotka said he regretted any disruption caused by the strike but said union memnbbers had been left with no other option. "As we have stated repeatedly, we remain ready and willing for meaningful talks," he added.
The permanent secretary at the DWP, Sir Richard Mottram, said the strike was unjustified as "there are other avenues the union can take". He was confident that a reasonable service could be maintained in the next 48 hours.
Paul Holmes, the Liberal Democrat spokesman for work said: "How can the DWP expect to have a well-motivated staff when, as well as low pay, the department has a poor record of training and high staff turnover."
He called for the government to minimise the disruption from the strike, and reach a settlement as soon as possible.
