About 15,500 nurses in Northern Ireland have begun strike action in a dispute over pay and patient safety.
The Royal College of Nursing represents about 9,000 nurses who walked out at 08:00 GMT – the first time the union has taken strike action in its 103-year history.
About 6,500 other nurses, who are members of Unison, walked out earlier.
It follows weeks of industrial action by other healthcare workers over the same issues.
The RCN strike is set to last for 12 hours while many Unison health staff members, including nurses and paramedics, are staging a 24-hour strike.
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Speaking at the Royal Victoria Hospital in Belfast, Royal College of Nursing (RCN) member Nuala Murray told BBC News NI: “This was incredibly difficult. I’ve been nursing for 37 years.
“This is so unprecedented for us to have to strike but nurses are so fed up, they’ve just had enough.
“They just feel really passionate about this. Their voices need to be heard.
“Their patients aren’t safe and they need to do something.
“We had no other choice but to strike in order for us to have our voices heard and that our patients are being nursed safely.”
Last night leaders of the five main political parties in Northern Ireland met the head of the NI Civil Service, David Sterling, and health permanent secretary Richard Pengelly in a bid to avert the strike action.
The letter to Julian Smith signed by the five party leaders said there was “collective support for the restoration of pay parity”.
Following their meeting with the Civil Service, the parties wanted to meet the Northern Ireland secretary, but the Northern Ireland Office said health remained a devolved matter.
Mr Sterling rejected the argument that he and his colleagues should restore pay parity on the grounds of public interest.
In a letter seen by BBC News NI, he told the party leaders: “This is not a question of simply needing ‘political cover’ or making a judgement on whether one particular pressing issue would pass a public interest test – the reality is that virtually any issue considered in isolation could do so.”
Mr Sterling said restoring parity would cause a £30m a year pressure on the Stormont budget and that cash could be instead spent on taking 120,000 people in Northern Ireland off waiting lists.
He added that the decision is a choice which should be taken by ministers.
Dr Tony Stevens, chief executive of the Northern Health Trust, told BBC News NI that while the health trusts will be delivering a “much reduced” service, they will be able to deliver services safely.
“Speaking on behalf of all of the trust chief executives, we along with the trade unions and staff have been working really hard in the last three days,” he said.
“We are satisfied that we will be delivering a much reduced service, but we will be able to deliver critical services safely during the next 24 hours.”
Analysis: Years in the making
By Marie-Louise Connolly, BBC News NI Health Correspondent
Of course this crisis didn’t just happen overnight, instead it has been years in the making.
The unions say that a failure to address workforce planning means there are almost 3,000 unfilled nursing posts.
Nurses say they have been putting up with these shortages for years and today a light is being shone on it.
2,800 unfilled nursing posts
According to the RCN, nurses’ pay within the health service continues to fall behind England, Scotland and Wales.
It argues that the real value of nurses’ pay here has fallen by 15% over the past eight years.
There are just under 2,800 unfilled nursing posts within the health service in Northern Ireland.
The RCN estimates that a similar level of unfilled posts exists within nursing homes.
The nursing vacancy rate in Northern Ireland is 13%, compared with about 11% in England and 6% in Scotland.
This means that for every eight nurses who should be working in Northern Ireland, one is missing.
Nurses are demanding better workforce planning.
Last year, the local health service spent £52m on agency nurses to fill these gaps in the workforce.
That money, the RCN argues, could be better managed to train and pay health service nurses.
Speaking to BBC Radio Ulster from the picket line, community nurse Karen Bowes said the strike was a “very sad day” in her 25-year nursing career.
“We didn’t come to this decision lightly and nurses have a heavy heart undertaking these steps,” the RCN member said.
“Our staffing levels are not safe – patients are not safe on a normal day,” Ms Bowes added.
She added that patients she had spoken in advance of the strike were “very supportive” of the action.
Ms Bowes urged politicians to step in and sort out the dispute.
Talks between management are continuing to ensure critical departments including cancer services and emergency departments are covered.
A spokesperson for Health and Social Care Board (HSCB) said major challenges were expected across all health and social care services on Wednesday.
It also advised that if patients or service users have not been contacted about their Trust then they should attend their appointment/ service as normal.
All emergency departments remain open, but “significant pressure” was expected within the departments.
“The priority will be on the treating emergency and life threatening conditions first. Patients with less urgent conditions may have to wait for lengthy periods,” said the spokesperson.
The heads of all of Northern Ireland’s health trusts have stated the current crisis in the service has been “years in the making”.
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