Hundreds of thousands of British teachers and public sector workers have swapped classrooms and offices for picket lines overnight.

Unions hope the protest 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.

However, 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 Thursday, 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.