Picket lines have been mounted outside job centres, benefit offices and Government buildings at the start of a 48-hour strike by up to 80,000 civil servants in a bitter row over pay.
The Public and Commercial Services Union said early indications suggested that the walkout at the Department for Work and Pensions was "solidly" supported.
The strike was called in protest at a three-year pay deal leading to average rises of 1% a year, which the union said was imposed without agreement.
Services to the unemployed, job seekers and benefit claimants will be affected by the strike, the union said.
Union general secretary Mark Serwotka predicted there would be "significant disruption" because of the nationwide walkout, which follows a strike by DWP workers last year.
The union has warned of growing unrest across the civil service over pay and has not ruled out further strikes in the next few weeks.
The DWP said it was confident services to the public would be maintained despite the strike.
Pickets outside a Government building near Trafalgar Square, in central London, handed out leaflets to visitors explaining why they were on strike.
The pickets accused the DWP of refusing to talk to the union about pay.
The pickets said more than half of DWP staff earned less than £17,700 and some as little as £12,500 a year.
Read More
MORE ABOUT