British union leaders and ministers clashed today over the impact of a strike by public sector workers who mounted picket lines outside schools, Government buildings, job centres and courts.

The Public and Commercial Services union (PCS) claimed it was the "best-supported strike" they have ever had, but the Government questioned the numbers taking part.

The Cabinet Office said early signs were that less than half of PCS members took part in today's strike.

Downing Street said there had been no significant delays at borders and no disruption at airports despite fears that travellers would be hit as immigration staff joined the 24-hour walkout.

Hundreds of thousands of teachers, lecturers, civil servants and other public sector workers were staging a 24-hour strike in protest at the Government's plans to increase their pension contributions and raise the retirement age.

The action has forced the closure of thousands of schools, courts and offices and disrupted Government services and travel.

PCS general secretary Mark Serwotka, said: "This is the best-supported strike we've ever had.

"The Government made a lot of the fact that after the strike ballot it was clear civil servants didn't support strike action, but today we can see that they have voted with their feet and sent a clear message to the Government that they will not tolerate these attacks on their hard-earned pensions rights and will fight the cuts that threaten to devastate our communities and jobs."

Members of the National Union of Teachers (NUT), Association of Teachers and Lecturers (ATL), University and College Union and the PCS are taking action today.

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 and we do regret that," he said.

"We had hoped to reach a settlement before the industrial action, but the Government isn't serious about talks."

Cabinet Office minister Francis Maude said teachers "absolutely don't" have to strike today, as talks are continuing.

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 were Parliament, the Royal Courts of Justice in central London, many courts including Westminster magistrates, and 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.

According to figures from the Department for Education, as of last night, around 4,640 local authority schools expected to be closed today, 3,888 partially open and 4,115 open.

Among academies, 223 were expected to be fully open, 204 partially open and 148 closed.

This equates to around a third open, a third closed and a third partially closed, Downing Street confirmed today.

Police leave has been cancelled in London, where union leaders and thousands of activists were taking part in a march, followed by a lunchtime rally in Westminster.

The Prime Minister's official spokesman described the strike as "unnecessary and premature", adding: "People are getting caught up in a semantic debate.

"There is this debate that is raging about unaffordable versus untenable. The fact of the matter is this was looked into very thoroughly by Hutton and he concluded that we needed to reform public sector pensions."

Asked if striking unions would be excluded from future talks, he said: "We want to have a constructive dialogue. We will continue to approach these discussions in that way."

Labour leader Ed Miliband said: "These strikes are wrong at a time when negotiations are still going on but parents and the public have been let down by both sides because the Government has acted in a reckless and provocative manner.

"After today's disruption, I urge both sides to put aside the rhetoric, get round the negotiating table and stop it happening again."

A Heathrow Airport spokesman said: "There have been no significant immigration delays at Heathrow so far today and we're pleased that contingency plans put into place by the UK Border Agency are working well. No flights at all have been disrupted due to industrial action."

The Cabinet Office said their figures showed that as of noon, more than three quarters of civil servants were not on strike, with just less than 100,000 taking part in the walkout.

Cabinet Office minister Francis Maude, said: "What today has shown is that the vast majority of hard working public sector employees do not support today's premature strike and have come into work today; I want to thank them all for coming in, ignoring the pickets and putting the public first."

He added he was "not at all surprised by the very low turnout for today's action".