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.

Click here to view picture from today's strikes

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, which affected large parts of the North-East and North Yorkshire.

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 Governments 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 per cent.

"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 dont 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 days 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.