Thousands of civil servants have gone on strike, disrupting court cases, driving tests and other services.
The union said the strike was well supported
The Public and Commercial Service (PCS) said up to 200,000 of its members throughout the UK walked out in a row about job cuts and pay.
The strike shut Woolwich Crown Court, interrupting the trial over the alleged 2005 London bomb plot, closed galleries at the Tate and disrupted tax offices.
The government said it valued staff but it had to ensure value for money.
A PCS spokesman told the BBC News website indications were that the strike had been at least as well supported as the last walkout in 2004, when up to 200,000 members took part.
Debates at the Welsh Assembly were cancelled, the reading rooms at the British Library closed, and both the Tate Modern and Tate Britain shut most of their galleries and displays.
A spokesman for HM Revenue and Customs said all its offices remained open and taxpayers would have been able to return self-assessment forms - for which the deadline is Wednesday.
He added: "There is the self-assessment helpline, and staff will be there to give advice."
But the union said managers had been forced to offer a much reduced service on one of its busiest days when hundreds of thousands of self-assessment forms are expected to be filed.
Earlier PCS spokesman Alex Flynn said: "The strike is being well supported, and where offices are open they are manned by a skeleton staff.
"If people want to get in to receive last-minute advice, they will not be able to. Where they have managed to keep offices open, they are being kept open by people who haven't dealt with the public for years."
The union wants guarantees that a decision to cut more than 100,000 civil service jobs will not be achieved through compulsory redundancies.
Earlier PCS head Mark Serwotka said those affected were not "faceless bureaucrats or high flying mandarins", but people providing everyday services that were taken for granted - many of whom were paid just above the minimum wage.
"Today's strike illustrates the depth of anger over the damage crude job cuts are having and the growing frustration over below inflation pay offers."
Cabinet Office minister Pat McFadden said there was "absolutely no need" for the strike and only one civil service union had chosen to do so.
He said civil servants were highly valued and did a "great job" for the public.
But he added: "At a time when the government is increasing investment in public services no organisation, including the civil service, can be immune from the need for change, both to ensure value for money for the public and to adapt to new technology."
But Labour MP John McDonnell, chairman of the PCS Parliamentary Group, said he would join people on the picket line to defend jobs and "end the scandal of low pay in the civil service".