The industrial action over pension reforms is believed to be the biggest for a generation.
Unions say that up to two million workers from across the public sector are taking part in the Day of Action.
The RCM has not balloted to strike, but earlier this week released a statement confirming its support for the action.
Jon Skewes, RCM director of employment relations and development, said: ‘We are absolutely opposed to our members' pensions being stolen from them.
‘If significant improvements to the existing package of proposals are not negotiated by the government, in good faith, we will be forced to ballot our members on industrial action early next year.’
He added that fewer threats from the government and more commitment to negotiations is needed.
The 24-hour industrial action is affecting hospitals, schools, airports, ports and government offices.
While maternity units remain open, thousands of non-emergency operations are being postponed.
However, patients will still be able to get emergency treatment and contingency plans are in place to protect A&E, cancer treatment and 999 calls are still being answered.
Brendan Barber,
TUC general secretary, said: 'On this unprecedented day, when 30 unions have members taking action together, we are sending a crystal clear message to the government that we are strong, that we are united, and that our campaign will go on until we secure justice and fairness for every public servant.
‘Let's be clear. Of course people never take industrial action lightly. Nobody wants to forsake a day's pay when the cost of living is high, and nobody wants to inconvenience the public and other working families.
‘But when unfairness is piled on injustice you are right to take a stand and I am proud to stand with every single one of you.’
Chancellor George Osborne this morning said that the strike is ‘not going to achieve anything’.
He continued: ‘So let's get back round the negotiating table, let's get a pension deal that is fair to the public sector, that gives decent pensions for many, many decades to come but which this country can also afford and our taxpayers can afford.’
