Firefighters, librarians and council staff are among those taking part from several trade unions, with rallies due to be held across the UK.
Thousands of pupils are set to miss lessons as many schools are closed.
But the Cabinet Office said it seemed most workers were at work as usual.
A spokesman for the cabinet office said the “vast majority” of workers did not vote for today’s action and said early indications suggested “most are turning up for work as usual”.
A spokesman said the vast majority of public sector workers had not voted for the strike.
The biggest issue in dispute is pay, after ministers froze public sector salaries in 2010 and introduced a 1% cap on pay rises in 2012 which remains in place.
Those taking part in the action include:
- Firefighters who are involved in a row over pensions and retirement age
- The Public and Commercial Services Union (PCS) – which representscivil servants, passport office workers and other public sector staff – in a dispute over pay, cuts to jobs, pensions and the privatisation of services
- Unison which represents, among others, workers in local government, healthcare, colleges and schools, and the GMBrepresenting, among others, workers who serve school meals, clean streets, empty bins, carers and school support workers, in a row over this year’s pay offer
- Unite members – including local government staff, council workersand teaching staff – who also dismissed the “insulting” pay offer
- The National Union of Teachers (NUT), whose general secretary Christine Blower said teachers “deeply regretted” taking strike action
- Members of the Northern Ireland Public Service Alliance, who work in a range of areas including housing, youth justice and libraries,striking over pay
- RMT members working for Transport for London are striking over pay and pensions
For Unite, Unison and the GMB the strike action covers workers in England, Wales and Northern Ireland but not Scotland, while the PCS covers all four nations. The FBU and NUT are England and Wales only.
Picket lines are expected to be mounted outside courts, council offices, job centres and fire stations across the country as well as the Houses of Parliament in London.
Unison boss Mr Prentis told BBC Breakfast workers had been left frustrated by pay freezes, adding that “enough is enough”.
“When Cameron brought in the two-year pay freeze, our local government workers, our members, had already had a one-year pay freeze.
“So they’ve had a three-year pay freeze and then a 1% increase when inflation has gone up by something like 20%,” he said.
Cabinet Office minister Francis Maude told the BBC the government had had to make “difficult decisions” to deal with the budget deficit.
BBC political correspondent Norman Smith
Strikes are meant to cause maximum pain for employers but in this case that doesn’t look like happening.
Why? Because far from causing the government much political grief, today’s strikes by public sector workers are actually rather politically convenient.
Of course there will be much ministerial condemnation of the disruption caused.
But at the same time Conservative ministers hope popular annoyance will buttress support for their plans for further action to curb the power of unions.
In particular, Prime Minister David Cameron is keen to include plans for a strike ballot threshold in his party’s next manifesto.
Today’s strikes enable ministers to turn up the heat on Labour by pressing them to condemn the industrial action being carried out by their big union supporters.
And they also expect widespread backing in the newspapers for their bullish stance.
So, while in public ministers will appear angered by the strikes, in private they may be good deal more relaxed.
Fire chiefs urged people to take extra care because of the walkout between 10:00 and 19:00 BST, the 15th round of industrial action in the Fire Brigades Union’s long-running row with the government.
TUC general secretary Frances O’Grady said: “Across the public sector workers are on strike today to say enough is enough.
“Year after year pay has failed to keep up with the cost of living.”
She said that public sector workers were on average £2,000 worse off because of the coalition government’s policies, saying “ordinary workers” should “get a fair share” as the economy started to grow.
PCS general secretary Mark Serwotka said the action by public sector workers was about “demonstrating that they’ve had enough”.
He told BBC Radio 5 live: “If that means that fire stations and schools are closed, and there are delays at airports and that people find that they are inconvenienced, whilst we regret the inconvenience, what we’re really trying to say is that everyone depends on our members’ services, so start paying them a decent wage.”
The strikes are going ahead despite the government arguing that they are based on ballots conducted some years ago, with low turnout from union members.
Speaking during Prime Minister’s Questions on Wednesday, Mr Cameron said the time had come for legislation setting a minimum turnout for a strike ballot, promising this in the next Conservative manifesto.
Under the current law, a strike can take place if it is backed by a simple majority of those union members who vote – regardless of the level of turnout.
Education Secretary Michael Gove also criticised the NUT’s ballot, which was held in 2012 with a turnout of 27%.
He told BBC Newsnight schoolchildren needed to be protected from what he said was “essentially politically-motivated industrial action”.
‘No end date’
But the union’s general secretary, Christine Blower, told the programme it was “perfectly legitimate”.
“We balloted for discontinuous action with no end date,” she said. “The end date will be when the dispute is resolved.”
A Labour Party spokesman said: “No-one wants to see a strike, not least because of the impact on children and parents.
“Instead of ramping up the rhetoric the government should get round the table, because both sides have a responsibility to stop it happening.”