Schools, hospitals, courts and transport were hit by the biggest strike in decades today as teachers, nurses and civil servants joined weather forecasters, botanists and nuclear physicists on picket lines.
Unions said early indications were that the walkout was being solidly supported and predicted that November 30 would go down in history as the biggest day of industrial action since the 1979 Winter of Discontent.
Hospital employees and workers on the Mersey tunnels were among the first to take action from midnight, setting up picket lines and holding up banners attacking the Government's pension reforms.
Early Government figures suggested that almost three in four schools were affected by the walkout, although that number could rise.
The Department for Education (DfE) said it believed that more than half of England's 21,700 state schools (58%) were closed, with a further 13% partially shut. Around 13% are open, the DfE said, while the rest are unknown.
Schools in Scotland, Wales and Northern Ireland are also expected to be affected.
Queues are expected to build up during the day at Heathrow airport, no ferries will sail to or from Shetland, and the Metro in Newcastle will not run.
Cabinet Office minister Francis Maude launched fresh criticism of the strike today, saying it was "inappropriate, untimely and irresponsible", especially while talks were continuing.
"Responsibility for any disruption which people may experience today lies squarely with union leaders. 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.
"This morning, 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 now estimate today that around three-quarters of schools in England will be closed or partially closed today. Council services such as refuse collection, street cleaning and libraries are also likely to be affected."
Employees at Birmingham Women's Hospital in Edgbaston were 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."
Among those walking out at the Mersey Tunnels 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."
Chancellor George Osborne urged unions to resume negotiations as he warned that today's strike over pensions will not "achieve or change" anything.
The Chancellor insisted that today's walkout by millions of public sector workers would make Britain's economy weaker, telling BBC Breakfast: "The strike is not going to achieve anything, it's not going to change anything.
"It is only going to make our economy weaker and potentially cost jobs."
Shadow chancellor Ed Balls laid the blame for today's walkout on Mr Osborne and Prime Minister David Cameron.
He told BBC Breakfast: "I think it's totally ridiculous, that all across the country businesses and families are being disadvantaged by the strike today.
"I think it's ridiculous, and it should have been sorted out.
"And the problem is, that there's some real unfairness in what the Government's proposing. The unions have to give some ground and I think they should, and they would have done, but the Government said weeks ago they're not going to talk any more, no more ground to be given."
He added: "It should have been sorted out. It was the Chancellor and the Prime Minister's job to give some ground and sort this out, they didn't, and that's why we end up with this strike today."
Unison leader Dave Prentis said unions had the public on their side, adding: "They know that public service workers are not asking for more - they just want the pension deal they were promised.
"Taking strike action is not an easy option, especially with Christmas just round the corner, but we will show Government ministers that we will not take this pensions tax lying down."
Mr Prentis told Radio 4's Today programme: "The last time we saw Treasury ministers and the Cabinet Office's ministers was November 2. They have not asked to see us since then.
"This idea that negotiations are continuing is just not true. We were told by Danny Alexander (Chief Secretary to the Treasury) there is a final offer, if you don't accept it it will be withdrawn - that's hardly the idea that negotiations continue."
Schools minister Nick Gibb said: "That is not true. As far as education is concerned, the general secretaries were in last week. They start negotiating again tomorrow, we are negotiating openly and honestly."
Chris Keates, general secretary of the NASUWT teaching union, one of those taking action today, accused ministers of misleading the public.
She said: "The only thing making the economy weaker is the Government's economic strategy. To continue to blame public sector workers isn't going to do anything to resolve the difficulties we're in.
"The Government constantly says get back round the table - that's all the NASUWT has wanted to do, but ministers haven't called a meeting since November 2."
Ms Keates added: "To keep claiming publicly that they want to negotiate, when ministers haven't called a meeting, I think that's misleading."
Members of the Rail Maritime and Transport union at the Royal Fleet Auxiliary joined picket lines at Portland, Falmouth, and on a joint protest with Public and Commercial Services union members in Portsmouth.
RMT general secretary Bob Crow said: "RMT transport workers in the North East and our members on the Royal Fleet Auxiliary, the service that supplies the Royal Navy fleet around the world in times of both war and peace, are standing shoulder to shoulder with millions of other public service workers involved in the action today.
"We are sending the clearest message to the Government that we will defend our pensions to the hilt and the outrageous demand that our members should work longer, pay more and get less has been thrown back in the faces of this Government of millionaire public schoolboys."
Russell Hobby, general secretary of the National Association of Head Teachers (NAHT), said: "The strike is there to send a message loud and clear about how people feel. That's an important achievement in itself, and actually what the coalition is doing to the six million taxpayers in the public sector is going to hurt the economy."
He warned that, from next year, spending by public sector workers is going to go down, which will affect Britain's economy.
"They will be starting to feel pay cuts, as pay is being capped, and due to increased pension contributions", he said.
Mr Hobby estimated that by the end of four years, school leaders will have seen their pay cut by 20%.
Mr Maude said later it was wrong of union leaders to maintain there had been no negotiations for weeks.
"There were formal discussions with the Civil Service unions only yesterday and there will be formal discussions with the teaching unions tomorrow and health on Friday.
"In addition, there are frequent informal contacts between the Government and the TUC.
"Contrary to what is being claimed this morning, talks are very much ongoing, intensive and making good progress - and it is misleading to claim otherwise.
"All of this underlines how indefensible today's strike is while these talks at scheme level are moving forward."