LONDON — British teachers and public service workers swapped classrooms and offices for picket lines today as hundreds of thousands walked off the job to protest pension cuts.

Airport operators warned of 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 say about 750,000 workers were expected to join the one-day strike, disrupting courthouses, tax offices and employment centers, as well as schools.

Thousands of union members were marching through London and other cities to demand that the government rethink its plans to curb public sector pensions.

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

London Mayor Boris Johnson, meanwhile, called for employment law changes that would make it harder for strikes to take place, telling the BBC that strikes take place even despite “very low” turnouts in strike ballots.

The government insists everyone must share the pain as it cuts $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.

Prime Minister David Cameron’s office said at least a quarter of schools in England and Wales were closed as a result of the strikes, and about another 22 percent were partially shut. But it said job centers, courts and government call centers were all operating as normal.

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.