Hundreds of thousands of British teachers and public sector workers swapped classrooms and offices for picket lines Thursday in what unions hope will be the first salvo in a summer of discontent against the Conservative-led government's austerity plans.

Airport operators had warned there could be long lines at immigration entry points because of walkouts by passport officers, but most of Britain's airports, including London's Heathrow and Manchester, said it was business as usual.

One union leader estimated more than 500,000 teachers and civil servants joined the one-day strike, affecting courthouses, tax offices and employment centers, as well as schools. The government estimated 100,000 strikers — although its tally did not include teachers, whose walkout closed or disrupted 11,000 schools in England and Wales.

"This is the best-supported strike we've ever had," union leader Mark Serwotka told Channel 4 News.

But the government disputed the claims, saying the strike wasn't as well-supported as the unions were making it out to be.

"Very few civil servants wanted this strike at all," Cabinet Office minister Francis Maude said in a statement.

Small groups of anti-capitalist protesters scuffled with police as the march neared Parliament, and were cordoned in by officers. Police said 41 people had been arrested over the past 24 hours, although the demonstrations were overwhelmingly peaceful.

The government insists everyone must share the pain as it cuts 80 billion pounds ($130 billion) from public spending to reduce Britain's huge deficit, swollen after the government spent billions bailing out foundering banks. It is cutting civil service jobs and benefits, raising the state pension age from 65 to 66, hiking the amount public sector employees contribute to pensions and reducing their retirement payouts.

But Britain is not Greece, whose crippling debt crisis has led to violent protests. Britain's economy remains weak as it emerges from recession, but the general mood is one of apprehension rather than anger.

Earlier in the day, Serwotka said that officials were ducking the real issues.

"It's time for the government to engage properly," he said. "It has shown it is unwilling to move on any of the central issues — that public sector workers will have to work up to eight years longer, thousands of jobs are at stake, lower pensions are set to cost three times as much, and pay is frozen while inflation soars.

Craig Phelan, a professor of modern history at Kingston University, says attitudes have changed dramatically since the 1980s, when unions took on Prime Minister Margaret Thatcher's government in grinding conflicts like a yearlong miners' strike — and lost. Then, more than 13 million Britons were union members. Now the figure is about 7 million.

"People see unions as something other, someone who wants to take their money, someone who wants to inconvenience them, someone who doesn't want to work as hard as they do," Phelan said.

It's a hard attitude for unions to overturn, and they are moving with caution.

London police said almost all of its civilian staff who answer emergency and non-emergency calls had walked out. The force drafted in police officers to cover the shortfall. Even employees at Prime Minister David Cameron's office walked out on the job — although officials put the figure at "fewer than five."

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."

"David Cameron has accused teachers of a lack of morality," she said. "Who really lacks morality? The thief or those who try to stop the thief?"