Strikes are taking place across the UK in a series of disputes with the government over pay, pensions and cuts, with more than a million public sector workers expected to join the action.
Teachers, council staff, firefighters and civil servants are among those taking part.
Unison general secretary Dave Prentis said workers were now £4,000 a year worse off than they were in 2010.
Prime Minister David Cameron has promised to tighten strike ballot laws.
Thousands of pupils are expected to miss lessons after schools have closed in parts of the UK as teachers strike.
Other workers to take industrial action include refuse collectors, dinner ladies and librarians, while unions rallies will also take place in towns and cities across the UK.
A spokesman for the Cabinet Office said the “vast majority” of workers did not vote for today’s action and said early indications suggest most are turning up for work as usual”.
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.