A strike by thousands of Jobcentre Plus staff in an escalating row over working conditions and claims of a "target-driven culture" was said to be solidly supported today.
The Public and Commercial Services union (PCS) said picket lines were mounted outside more than 30 call centres across the country where staff walked out for 24 hours.
The union said the workers, many of whom had never taken industrial action before, were receiving "massive" support from members of the public.
Callers to the centres were being told to ring back tomorrow when the strike was over, said the union.
Today's action follows a two-day strike in January by more than 2,000 workers in Jobcentre Plus's seven newest contact centres, who complain of being forcibly moved from processing benefit claims to handling inquiries by phone.
The union said it wants to improve the levels of customer service in call centres, end a target-driven culture, particularly by changing the way "unrealistic" average call times are used, and introduce proper flexible working arrangements.
PCS general secretary Mark Serwotka said: "The workers involved in this dispute are some of the lowest paid in the Civil Service who are being forced to take action to improve the service they provide to some of the most vulnerable people in society, in the face of unacceptable working conditions and an obsession with computer-driven targets.
"The truth is, staff are monitored every minute of the day. The computer dictates start and finish times and tells them when to go for a break, with staff hauled up if they are 40 seconds late back or go over the time allowed for a call. Toilet breaks are monitored and constantly questioned.
"These call centres are a vital lifeline for members of the public when they need to claim benefits, when they're sick or disabled or need help getting back into work. Inquiries are often complicated and many callers are understandably desperate and upset - some of them have no-one else to turn to.
"Our members are not numbers, and neither are the unemployed, and they want to help people, but often they're encouraged to just get the caller off the phone as quickly as possible and this cannot be right."
PCS president Janice Godrich said: "Unemployment is rising and we're seeing swingeing attacks on welfare. It's unacceptable that standards of service in Jobcentre Plus are falling as a result of staff being subjected to an unachievable target culture that is given priority over delivering vital public services."
A Department for Work and Pensions spokesman said: "We are disappointed that, despite three-quarters of staff across the centres having not voted to go on strike, the PCS have decided to take industrial action.
"The contact centre staff at DWP have good terms of employment including generous holidays, and have a good amount of flexibility. But we have to ensure that our service is available when our customers, who include some of the most vulnerable people in the country, need us.
"We use performance measures to ensure that performance and productivity are high, and we deliver value for money for the taxpayer."