Picket lines were mounted outside schools, government buildings, jobcentres and courts today by striking public sector workers in the biggest outbreak of industrial unrest in Britain in years.
Union leaders said early indications were that the 24-hour walkout was being strongly supported, forcing the closure of thousands of schools, courts and offices and disrupting government services and travel.
Members of the National Union of Teachers (NUT), Association of Teachers and Lecturers (ATL), University and College Union and the Public and Commercial Services union (PCS) went on strike in a bitter row over plans to increase their pension contributions and raise the retirement age.
The PCS said it was encouraged by support from its members and announced that it had recruited 2,000 extra members in the past few weeks.
"We are expecting the best supported strike we have ever seen," said one official.
Kevin Courtney, deputy general secretary of the NUT, said the early indications were that "large numbers" of schools were affected by the action, around 80%.
"We realise that's very disruptive for parents," he said, "and we do regret that.
"We had hoped to reach a settlement before the industrial action, but the Government isn't serious about talks."
He added that the Government needs to talk seriously about the issues the unions are raising, and not simply about "how to implement their agenda".
Cabinet Office minister Francis Maude said teachers "absolutely don't" have to strike today, and should not because talks are still ongoing, with another meeting between the Government and trade unions due next week.
He told BBC Breakfast: "It's absolutely unjustifiable for parents up and down the country to be inconvenienced like this, forced to lose a day's work when they're trying to go out to work to earn money to pay taxes that are going to support teachers' pensions, which will still, at the end of this, be among the very best pensions available."
Among the buildings being picketed was Parliament, with strikers saying they hoped some left-wing MPs would refuse to cross.
Strikers also picketed the Royal Courts of Justice in central London and many courts including Westminster magistrates.
Unions were also targeting the headquarters of the education and business departments.
Immigration officers at ports and airports across the UK joined the strike, with the prospect of long delays for travellers returning to the country.
The PCS raised doubts that managers being drafted in to cover for strikers would be able to process passports or immigration papers quickly enough.
Police leave has been cancelled in London, where union leaders and thousands of activists will take part in a march, followed by a lunchtime rally in Westminster.
The TUC said today that millions of public sector workers were having to pay for the deficit they did nothing to cause.
General secretary Brendan Barber, who is visiting picket lines in the South West, said public sector workers' pay has been frozen while it was "bonuses as usual" in the financial sector.
"This is gold standard for unfairness."