Thousands of public sector workers have taken to the picket lines in Northern Ireland in a mass strike against spending cuts.
Bus and trains are not running, many schools have closed and medical appointments have been postponed due to the 24-hour action.
The strike has already been marked by an angry row between ambulance workers and management after bosses activated emergency protocols late last night to prevent crews joining the walkout.
The Northern Ireland Ambulance Service declared a "major incident" in response to the number of staff threatening to take industrial action, claiming that cities like Belfast and Londonderry would have been left without any ambulance cover.
The move, which compelled staff to turn up for work, has been condemned by unions as "cynical".
The NIAS had reached an initial agreement with trade unions with the aim of providing cover for only emergencies throughout today, but the service declared the incident after it said it had been "inundated" with calls from staff saying they would not be working.
Unions have insisted the situation did not warrant the declaration of a major incident.
Patricia McKeown, Unison regional secretary, accused the NIAS of engaging in a "strike-breaking manoeuvre" which was a "complete breach of faith".
"Its decision to declare a major incident across the whole of Northern Ireland two hours in advance of strike action is provocative," she said.
"It is not a smart move if that was the intention. Instead it is likely to provoke further action.
"There are no indications that such a major incident exists. This is strike breaking and intimidation."
Kevin McAdam, Unite national officer, said: "The trade unions know now the facts on the ground: that there was no major incident, rather there was an inability and unwillingness for management to effectively manage the strike.
"We are unhappy and disgusted by the NIAS cynically exploiting this part of the emergency cover understanding to thwart the industrial action."
But the NIAS said that without taking the step, cities and towns including Belfast, Londonderry, Omagh, Enniskillen and Lisburn would have been without ambulance cover from midnight.
NIAS spokesman John McPoland said: "NIAS has exhausted all alternative contingency options and lives would be at risk if we allowed the situation to deteriorate further."
Union leaders have warned that today's action will be the biggest in many years.
Workers are angry at Stormont budgets that have cut millions of pounds from public spending; a voluntary redundancy scheme to reduce the Civil Service by 20,000; and a proposed cut in corporation tax in Northern Ireland that would see millions shaved off public finances to fund a tax incentive for businesses.
Public transport services across Northern Ireland have been cancelled. Operator Translink has been unable to run any buses or trains and the police have given motorists permission to use many bus lanes in an attempt to avoid traffic logjams.
Members of the Unison, Unite, Nipsa, INTO and GMB unions working in health, education, the Civil Service, transport and a range of other public services are striking.
The fate of the redundancy scheme, next year's Stormont budget and the devolution of corporation tax powers from Westminster to Belfast are shrouded in uncertainty after a political row over welfare reform threatened the Stormont House Agreement in which those proposals were included.