The unions has accused ministers of refusing to negotiate over its economic and social policies.
Unite General secretary Len McCluskey said:
Our members at HCA are showing solidarity with their HCA colleagues in PCS against the savage attack by the coalition on public services and those dedicated staff that deliver them.
The anger is growing at the negative dead end economic and social policies of this right-wing government - from the easy ride that the City elite receives from George Osborne to the gross unfairness of the hated bedroom tax which will hit 650,000 people.
He said: "I can reassure the public that the Government has done everything we can to minimise disruption [today]. Rigorous contingency planning is in place to try and limit the impact of the strike action and to ensure that affected public services operate as effectively as possible."
A rally will be held in Westminster while the Budget is being delivered by Chancellor George Osborne today.
Union members are protesting as part of a long-running dispute with the Government over pay and pensions, with thousands of public sector workers staging a 24-hour strike today.
This is not a one-day protest, this is the start of a rolling programme of walkouts and disruptive action to put pressure on a government that is refusing to talk to us.
We warned more than two years ago that austerity wouldn't work and we were right. There is an alternative to cutting the living standards of hard-working public servants and our campaign is designed to make the case loud and clear.
Civil and public servants are working harder than ever to provide the services we all rely on but, instead of rewarding them, the Government is imposing cuts to their pay, raiding their pensions and trying to rip up their basic working conditions.