Union leaders said they were "overwhelmed" by support after a strike by public sector workers caused widespread disruption to schools, councils and the fire service.
The walkout was sparked by a series of bitter disputes with the Coalition over pay, pensions, jobs and spending cuts.
The Government said fewer than half a million took part in today's action, but the six unions which organised the strike described the figure as "laughable".
The GMB union said more people were on strike than just union members, putting the total at well over a million.
ITV News Political Correspondent Emily Morgan reports from Birmingham, where thousands of angry workers took to the streets:
Schools across England and Wales closed and there was disruption to council services, museums and galleries, the fire service, courts, driving test centres, libraries and jobcentres.
Around a fifth of schools in England were forced to close, according to the Department for Education.
A spokeswoman said:
The NUT has tried to cause as much disruption for children and their families as possible - but thanks to the dedication of many teachers and staff who turned up for work, just 21 percent of schools were closed today.
Unison said the strike was particularly well supported in the North East, Wales and East Midlands where the union said most council offices were closed.
Unison general secretary Dave Prentis said:
It is a massive decision by local government and school support workers to sacrifice a day's pay by going on strike, but today they are saying enough is enough.
ITV News Correspondent Rupert Evelyn reports from Somerset, where he met those who took part and those whose day was disrupted:
The Prime Minister and other senior politicians attacked the strikes, arguing that they were based on ballots conducted some years ago which saw low turnout from union members.
The Conservatives are drawing up plans to change employment law so that a threshold of those balloted would have to be reached before industrial action could be held.
But Unite said no Tory Cabinet member achieved a 50% voting threshold in the last general election.
It is utter hypocrisy for the Government to talk about mandates for trade unions when not a single member of the present Cabinet would have been elected using the same criteria.
Cabinet Office minister Francis Maude said the Government's official estimates were that fewer than half a million took part in the strike action. He said:
Within the Civil Service, there has been the lowest recorded turnout for a national strike. Every jobcentre opened, the majority of children went to school as normal and fire services continue to operate with robust contingency arrangements in place.