Hundreds of thousands of public sector workers and teachers in Britain took part in a one-day strike Thursday, protesting against proposed changes to their pensions.

"Our members have voted with their feet and supported the strike," said Mark Serwotka, Public and Commercial Services Union secretary, at an afternoon rally. "We are in it together with public sector workers, students and pensioners defending everything we have fought for for generations."

But the exact number of demonstrators who took to the street is disputed.

One union leader told The Associated Press that more than 500,000 teachers and civil servants joined the one-day strike, affecting courthouses, tax offices and employment centres, as well as schools.

The government's estimate was lower. It said roughly 100,000 strikers hit the streets but that did not include teachers, whose walkout closed or disrupted 11,000 schools in England and Wales.

Small groups of anti-capitalist protesters clashed with police as the march neared the parliament buildings in London, and were cordoned in by officers. Police said 41 people were arrested throughout the day, although the demonstrations were overwhelmingly peaceful.

Travellers were warned of bottlenecks at U.K. airports because of strike action by the UK Border Agency. Airline passengers were told to expect delays at passport control, but most of Britain's airports, including London's Heathrow and Manchester, said it was business as usual.

Prime Minister David Cameron's office said job centres, courts and government call centres were all operating normally.

The government wants to increase the age at which workers would be able to collect a full public sector pension — from 65 to 66 by 2020.

Cameron has said reform is essential to stop the pension system from "going broke."

Workers say the proposals will mean they have to work longer and contribute more, all for a reduced pension.

In central London, thousands of union members listened to speeches and demanded the coalition government rethink its plans to curb public sector pensions.

While some British trade unions — such as those representing London subway drivers — have a reputation for frequent strikes, their public sector counterparts are traditionally moderate. There has not been a national strike by teachers since the 1980s, and for one of the unions, the Association of Teachers and Lecturers, Thursday was its first strike in its 127-year history.

Their leaders say they have no choice, saying their members worked many years for modest pay, on the promise of a solid pension, and accuse the government of reneging on that deal.

Helen Andrews of the National Union of Teachers told a rally in Manchester that teachers were being asked to "pay more, work longer, get less."

Thursday's walkouts are the first salvo in what unions hope will be a summer of discontent over the government's austerity plans.

With files from The Associated Press