Up to two million state workers have walked out across the UK, in a dispute with the government over pension reforms.
Unions are billing it as the biggest strike in a generation.
Teachers and nurses are among those taking part in the action, forcing schools to be shut and hospitals to cancel operations.
Unions say the pension changes will force public sector staff to work for longer before they can retire – and pay more for pensions, which will be worth less.
Immigration officers are also among those striking.
At London’s Heathrow airport, there had been fears that arriving passengers would be stuck in long queues at passport control.
The government has brought in replacement staff and there does not appear to have been many delays for travellers so far.
Britain’s Chancellor George Osborne has condemned the strike.
The government, trying to turn around a debt-laden economy, maintains reform is needed.
It says people are living longer and public sector pensions are unaffordable.