The biggest strike for over 30 years got under way today with schools, hospitals, courts, transport and government set to be hit by a walkout involving up to two million workers.
Nurses, paramedics, cleaners, porters and receptionists at Birmingham Women's Hospital were among those who walked out at midnight in the row over pensions.
Picket lines were being mounted outside thousands of schools, hospitals, jobcentres, courts and other buildings, while more than 1,000 rallies and other demonstrations will be held across the UK.
John Longworth, director general of the British Chambers of Commerce, said the strike was "irresponsible and reckless", adding: "Trade unions are living in a bubble and ignoring the fact that Britain has to make its way in a competitive world."
Weather forecasting staff at the Met Office, workers at the Royal Fleet Auxiliary, catering employees in the Commons and museum curators are among those due to join the strike, while more than 1,000 rallies and demonstrations will be held in towns and cities across the UK.
Queues are expected to build up at airports including Heathrow, no ferries will run to or from Shetland, the metro in Newcastle will not run, and the Mersey tunnels will close, disrupting the 80,000 motorists who drive through every day.
Union leaders will travel to towns and cities across the country in a show of support for the strikers, many of whom will be going on strike for the first time.
Workers ranging from lollipop ladies and refuse collectors to head teachers and nuclear physicists will take action in the biggest day of industrial unrest since the 1979 Winter of Discontent.
TUC general secretary Brendan Barber will tell a rally in Birmingham that the Government had scrapped a bankers' bonus tax and replaced it with a "teachers', nurses' and lollipop ladies' tax".
He will say that no one takes industrial action lightly or wants to inconvenience the public, adding: "When unfairness is piled on injustice you are right to take a stand.
"Ministers keep saying that all they want is to secure the long-term affordability of public service pensions. The brutal truth is simply this - that the living standards of millions of low and medium-paid public service workers are being hammered in the name of reducing the deficit.
"The cuts are beginning to scythe through our public services, more and more jobs are under threat, and as the pay freeze bites - while inflation roars ahead - real wage cuts are making it harder than ever to make ends meet.
"We found out yesterday that the Government is to step up the attacks on public sector staff with a continuing pay cap and pay variation at a local level - and I don't think they mean better.
"This is on top of an increase in the state pension age for anyone under 40 - a toxic triple that will do nothing to get the economy moving again, but will simply hit consumer confidence."
A poll of almost 2,800 people for the TUC showed that most believed the average public sector pension was £14,000 a year instead of the actual figure of £5,600.
Mr Barber said union members on strike today should take "great comfort" from the survey.
The Government told unions that the strike will achieve "nothing" as it issued a fresh warning about the impact of the walkout at airports.
Cabinet Office minister Francis Maude said a "significant" number of civil servants had volunteered to cover for striking Border Agency staff, including a "considerable" number from the Ministry of Defence.
The Prime Minister's official spokesman said: "There are going to be queues but we have proper contingency plans in place and we have been training people to do what we can to mitigate the impact. The strike action will achieve nothing - it would be far better to continue with the talks."
In Scotland, Labour MSPs have pledged their support and will not cross picket lines at Holyrood, while Labour AMs in Wales will not cross picket lines at Cardiff Bay.
A number of Labour MPs will not cross picket lines being mounted outside Parliament in Westminster, although the GMB union said it wanted them to be in the Commons during Prime Minister's Questions to voice their support for the strike.
Belgian carrier Brussels Airlines added to the list of airlines that have axed flights by cancelling one of its Heathrow to Brussels services today.
Middle East airline Etihad Airways and Greek carrier Aegean Airlines had already announced they were scrapping some services to and from Heathrow, while BAA has asked airlines to fly planes into the airport only half full while the strike is on.
Union leaders said the 1% pay cap on public sector workers for two years after the current two-year freeze ends, and bringing forward to 2026 the date on which the state pension age will increase to 67, both announced by the Chancellor in his autumn statement, made it more difficult to resolve the dispute.
Paul Kenny, leader of the GMB, said: "As well as the shameful unfairness of further pay restraint on already hard-pressed public sector workers, the Chancellor's announcements will push the possibility of a pensions deal further away."
But Mr Maude said the strike was "inappropriate, untimely and irresponsible", especially while talks were going on.
"We have listened to the concerns of public sector workers and that is why at the beginning of this month we put an improved offer on the table.
"The offer ensures that public sector pensions will remain among the very best available while also being fair and affordable to taxpayers.
"While discussions are continuing, I would urge public sector workers to look at the offer for themselves rather than listening to the rhetoric of their union leaders.
"These are the sort of pensions that few in the private sector can enjoy. In the meantime, I want to reassure the public that we have done everything we can to minimise disruption.
"Rigorous contingency planning is in place across all sectors to try and limit the impact of the strike action and to ensure that key public services remain open.
"However, we estimate that around 90% of schools in England will be closed and passengers may face longer than normal waiting times at airports and ports. Council services such as refuse collection, street cleaning and libraries are also likely to be affected.
"There will also be an impact on health services and some organisations have had to reschedule elective surgery and outpatients appointments so that urgent cases can be prioritised.
"Emergency and critical care services will be operating normally and 999 calls will be responded to as usual.
"Let me say again that the timing of this strike is indefensible and wrong.
"Union leaders should be responding in a responsible manner to reach agreement. A strike will not achieve anything other than causing inconvenience to hard-working people at a time when we are trying to get the economy back on its feet."
The Mersey Tunnels in Merseyside were closed just after midnight as workers became some of the first public sector employees in Britain to strike.
Among those walking-out was Inspector Russ Aitken from Mersey Tunnel Police, who is taking industrial action for the first time in 35 years.
"I feel quite strongly that I need to come out on strike," he told BBC Radio 5 Live.
"I feel angry that I'm paying a 50% increase in pension contributions and I feel angry that I'm going to have to work longer and at the end of it get less."
Asked what he hoped striking would achieve, the local government worker added: "Hopefully the Government will change its position. The situation was made by the Government and the bankers and the people who are asked to pay the price is public sector workers."
Also joining the strike was Unison member Roger Irvine, who works for Mersey Travel - the company which runs the Kingsway and Queensway road tunnels. He told 5 Live strike action was the "last resort".
"We feel we have no other choice," he said.
"I think people in the public sector are getting very angry that the public sector seems to be the punch bag for the banking sector."
Employees at Birmingham Women's Hospital in Edgbaston were also among the first public sector workers to walk out at midnight, setting up picket lines which will be manned for the next 24 hours.
Unison president Eleanor Smith, who is a theatre nurse at the hospital which employs around 400 union members, said: "This has not been a decision that I've taken lightly. I have been a nurse for 30 years and this is the first time I have been compelled to take this action because of the Government.
"The Government wants us to work longer, pay more and at the end get less. How fair is that?"
The nurse added: "I came into the public sector not for great wages but for a pension. Now this pension which I was relying on is going to be taken away - not totally, but considerably reduced. I get the impression the Government doesn't like the public sector."
Brendan Barber, general secretary of the TUC, said the public sector was "absolutely under attack" by the Government, adding that the day of action was completely justified.
He told ITV's Daybreak: "There comes a time when people really have to stand up and make a stand. With the scale of change the Government are trying to force through, making people work much, much longer and get much, much less, that's the call people have made."
Schools Minister Nick Gibb branded the action unnecessary, saying the Government was continuing negotiations with the unions.
"I can't understand why the unions are causing huge disruptions to millions of people across the country, with parents forced to pay expensive childcare costs or take a day off work," he said.
"These reforms are necessary so we can sustain these good quality pensions in the long term."