Hundreds of thousands of public sector workers are walking out once again over their public sector pensions, but the government says further resistance is futile.
Cabinet Office minister Francis Maude is refusing to reopen pension talks after last autumn's 'final offer' from ministers.
Some unions accepted the coalition's concessions to its public sector pensions arrangements, securing outline agreements in the run-up to Christmas.
The government had offered more favourable accrual rates, which determine the pace at which pension pots grow. Health and education workers will receive a "revised revaluation factor" of consumer price index inflation plus 1.5% and 1.6% respectively.
Some unions have refused to back down, however. PCS members voted to reject the government's offer, leading to today's strike. Other unions participating in today's industrial action include Unite, the University and College union and the Immigration Services union.
Today's strike is the biggest since the massive walkout on November 30th last year. Up to 400,000 workers could participate in the industrial action.
"It is very disappointing that a handful of unions insist on carrying on with futile strike action which will benefit no one," Cabinet Office minister Francis Maude said.
"We would urge these union leaders to reconsider their position. Pension talks will not be reopened and nothing further will be achieved through strike action."
The government set out its final proposed agreements in March after a year of negotiations. Chancellor Georg e Osborne had triggered outrage in his 2011 Budget by confirming plans to cap public sector pay at one per cent for two years, after the current pay freeze ends, and take steps to 'rebalance' pay levels across the country.
"Public sector workers are being asked to work a bit longer and pay a bit more, but they will continue to get a guaranteed pension which is index-linked and inflation-proofed," Mr Maude added.
"Most staff on low and middle incomes will receive a pension at retirement as good as what they expect today, and for many it will be even better."
The Unite union told its members that the "fight" with the government was "much wider" than just pensions, however.
"The NHS break-up and cuts, defence cuts, pay cuts and now the threat to regional pay are all part of this government's war on the nation's public services," it stated.
Protesting strikers will march through central London this lunchtime before gathering in Westminster's Methodist Hall for a rally.
A separate march by off-duty police officers opposing cuts to the Home Office's budget will also take place today.
"The march is the only way that police officers can demonstrate their anger," Police Federation chairman Paul McKeever explained.
"We have been inundated with messages of support from our colleagues who cannot attend due to their work commitments.
"The officers marching are doing so in their own time, that's how strongly they feel. Some will have been travelling since 4am to ensure they can take part."
Police officers from all 43 forces in England and Wales will attend the march. It is the biggest single protest by the police since a 2008 pay dispute with then home secretary Jacqui Smith, who had reneged on a pay deal.