Hundreds of thousands of health workers including those in Huddersfield, Kirklees and Calderdale have walked out on strike in protest at the Government's decision not to give them a recommended 1% pay rise.
Midwives, nurses, paramedics, ambulance staff, and hospital porters and cleaners mounted picket lines across England from 7am for four hours, while action will be taken later today in Northern Ireland.
Several trade unions are involved in the action, including those representing nurses, paramedics, hospital porters and ambulance crews as well as the Royal College of Midwives (RCM).
The strike, the biggest of its kind for over 30 years, will be followed by other forms of industrial action for the rest of the week, ahead of a national protest in London on Saturday organised by the TUC under the banner Britain Needs A Pay Rise.
The dispute involves over 400,000 NHS staff, who have been hit by pay freezes or below inflation rises since the coalition came to power in 2010.
Unite said a poll it commissioned showed public support for the industrial action as well as backing for a decent pay rise.
Contingency plans have been worked out to deal with the strike and union members will deal with emergencies. But a row broke out after it emerged that military personnel and the police will be deployed by the London Ambulance Service.
Unions said they had been kept in the dark about the move.
Unions are protesting at the Government's decision not to accept the independent pay review body's recommendation to award a 1% pay rise to all staff. Instead, ministers took the "divisive" decision to only award a 1% pay rise for those on the top of their pay band, which unions say has denied it to 60% of NHS workers.
Christina McAnea, national officer of Unison, said: "This is the first time in 32 years that NHS workers take industrial action over pay, and for many, it will be the first time. Up and down the country, hundreds of thousands of workers are out fighting for fair pay and for the NHS."
TUC general secretary Frances O'Grady, who joined a picket line near Euston in London, said: "NHS staff are always reluctant strikers - there hasn't been a national strike over pay in the health service since 1982 - and they will do everything they can to protect patients in their care. But morale has hit rock-bottom.
"Not only have NHS staff faced year-on-year cuts in the relative value of their pay, ministers have turned down the recommendations of the independent pay review body, even though it called for an affordable, below-inflation pay rise.
"It is no surprise that the NHS is finding it hard to recruit and retain staff as they find themselves squeezed between falling living standards and covering up for NHS cuts."
Cathy Warwick, chief executive of the RCM, said: "At a time when MPs are set for a 10% pay hike, we're told that midwives don't deserve even a below-inflation 1% rise. And politicians wonder why the public does not afford them more respect.
"It feels to a great many people, including midwives, that there is one rule for them and another rule for everybody else.
"The independent panel of experts who advise the Government on NHS pay recommended a 1% pay rise for midwives, nurses, paramedics and other NHS staff. Unfortunately, Health Secretary Jeremy Hunt rejected that recommendation, and he and the employers decided that midwives and others won't get a pay rise this year."
NHS England's chief nursing officer Jane Cummings said: "We know that NHS staff are highly professional and always wish to put patients first. I am sure they will think very carefully before taking strike action to ensure the safety and care of patients is not put at risk.
"As a nurse, I know that Monday mornings are often extremely busy for the NHS and it may be busier than normal this Monday because of the strike action being taken by some staff. As ever, the safety and care of patients is our top priority and we have robust plans in place to cope.
"If necessary, the most urgent cases will be put first and we would ask the public to help, for example, by only calling an ambulance if it is a life-threatening situation."
London Ambulance's director of operations, Jason Killens, said the service was expecting to be under "significant pressure" during the strike, adding: "While we recognise the right of our staff to take action, we must also do all we can to continue to provide a safe service to Londoners.
"We have plans in place to ensure we reach the most seriously ill and injured patients as quickly as possible and these include additional support from hospital doctors and nurses, private ambulance crews, the police and the military.
"In the event of a major incident we are confident that our professional and dedicated healthcare professionals would return to work and this has been agreed by our local union representatives.
"We will be sending a response to all patients in a life-threatening situation but other people may be referred to NHS111, given clinical advice, provided with alternative transport, may wait longer or will be asked to make their own way to hospital.
"As always we urge Londoners to call NHS111 and only call 999 for an ambulance in a genuine emergency so we have ambulance crews free to respond to life-threatening emergencies."
Dave Prentis, Unison's general secretary said: "Our NHS members don't take action often or lightly. For many, it will be the first time they walk out as the last action over pay was 32 years ago.
"But what NHS workers also don't do is walk out on patients. Safety is paramount and we have been working with employers to ensure urgent patient care is not compromised. Patient safety is paramount and life and limb cover will be provided.
"I reiterate my offer to Jeremy Hunt to reconsider his pay policy and negotiate with trade unions to avoid more industrial action as we get closer to the winter pressures."
Health Secretary Jeremy Hunt said: "We absolutely will talk to the unions, we've been prepared to talk to them throughout this. We will talk to them if they're prepared to look to reform the system of increments, which is unclear and unfair.
"I recognise frontline staff do a magnificent job in the NHS. We have had more than 650 NHS volunteers who are willing to put themselves in harm's way to help tackle the Ebola crisis in West Africa.
"We have magnificent people.
"We're offering them all a one per cent rise. The majority of NHS staff get an automatic 3% increment but we can't afford to give a one per cent rise to people already getting that.
"We're coming through a very difficult period with the tragedy of Mid-Staffordshire. We now have got 5,000 more nurses in just a year. We're recruiting nurses like crazy at the moment and to put that in reverse would not be the right move at the moment.
"The most important thing here is doing the right thing for patients. It would be irresponsible for any health secretary to accept a pay package that means the NHS has to lay off nurses."