Thousands of public sector workers across Northern Ireland are participating in the most widespread action since the Ulster Workers' Council stoppage in May 1974 which forced the fall of the first power-sharing government.
Jimmy Kelly, regional organiser at the Unite union, said: "We have been brought to a situation where our members across health, education, local government and transport are really declaring that enough is enough."
Many local council services have been cancelled.
Trade union rallies and pickets have been organised across the region.
The main rally is at lunchtime in Belfast while others are planned for Newry, Londonderry, Downpatrick, Omagh, Ballymena, Portadown, Magherafelt and Cookstown.
The civil service, health, education and transport are areas affected, although doctors and nurses are not taking industrial action.
There will be cover for those requiring emergency services.
The Northern Ireland Public Service Alliance, representing members across the civil service, will alone have more than 45,000 workers on strike and stage 160 pickets.
All of Northern Ireland's 1,200 schools face disruption to transport and catering services, and with most of the teaching unions striking, it is likely that a majority will be forced to close their doors.
In the health sector, emergency services are to be maintained and GP services and ambulances will be kept running despite disruption caused by industrial action.
The private sector has been critical of the day of action, but assistant general secretary of the Irish Congress of Trade Unions Peter Bunting defended the strikes.
"I think today is necessary because the public sector workers, and workers in general, are facing in many cases very, very serious reductions in their terms and conditions of employment," he said.
"And it will impact on the quality of life of everyone."
Mr Bunting said cuts were removing money from the economy and would impact on both the public sector and the private sector.
Video: Strike action underway
He added the significance of the strike action went further than protecting public sector pensions.
Mr Bunting said union members were also mobilising against wider Government cuts which he said had failed to bolster the troubled economy.
"It is also putting down a marker that we will oppose the austerity cuts that have not worked and will not work," he said. "We need a Plan B."
Video: Further union strikes inevitable
He cited economic projections which he said showed Government policies had failed to promote real economic growth.
Mr Bunting said the Government should look elsewhere for revenue and said £123 billion was lost in tax evasion, tax avoidance and through unclaimed tax.
"It is just incredible what is happening here," he said, arguing that a policy of cuts was "threatening the future of our children".
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