BRUSSELS — A national strike in Belgium over pay and working conditions has led to the cancellation of all flights in and out of the country, halted public transport and prompted blockades outside factories, threatening to bring the country to a standstill on Wednesday.

The 24-hour strike began at 10 p.m. Tuesday, with Belgium’s three main workers’ unions galvanizing their nearly four million members — in a country of about 11 million people — to stop working.

Government workers also joined the strike, prompting a number of public schools, nurseries and sports facilities to close. Prison guards went on strike and were replaced by police officers in most prisons.

“What we want is to tell employers, whoever they are, that we’re sick of them putting all the dough that we create in their pockets. It’s time to give some of it back to the workers,” said Robert Verteneuil, president of the General Federation of Belgian Labor, a socialist trade union with about 1.2 million members, on public radio Wednesday morning.