LONDON (AP) — British teachers and public sector workers swapped classrooms and offices for picket lines Thursday as hundreds of thousands walked off the job to protest pension cuts.

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

Unions estimated that up to 750,000 teachers and civil servants joined the one-day strike, disrupting courthouses, tax offices and employment centers, as well as schools. The government said the figure was lower.

Thousands of union members marched through London and other cities to demand that the government rethink its plans to curb public sector pensions. Small groups of anti-capitalist protesters scuffled with police as the march neared Parliament, and were cordoned in by officers. Police said 37 people had been arrested for offenses including possession of drugs, criminal damage and breach of the peace.

Thursday’s walkouts are the first salvo in what unions hope will be a summer of discontent against the Conservative-led government’s austerity plans.

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

Prime Minister “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?”

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.

Unions say their members work many years for modest pay on the promise of a solid pension, and accuse the government of reneging on that deal.

The two sides gave widely differing assessments of the level of disruption.

Schools were the hardest hit. The government said 11,000 schools were closed or partially closed, more than half the total of 21,000 in England and Wales.

The Metropolitan 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 fill the gap.

The government said job centers, courts and government call centers were all operating as normal, and “less than half” of civil servants in the striking unions had stayed away.

But Mark Serwotka, head of the civil service PCS union, said 85 percent of his members were out on strike — including some members of Cameron’s 10 Downing Street.

Cameron’s office said “fewer than five” Downing Street staffers had stayed away, and the strike had had “minimal impact” on the public.

Cameron spokesman Steve Field said passengers were not suffering serious delays at airports or ports, despite walkouts by some border agency staff.

“The early indications, and it is quite early still, are that the turnout is lower than perhaps the unions had claimed,” Field said.

Serwotka insisted the turnout for the strike was high.

“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.”