Greek public sector workers have walked off the job in a 24-hour strike to protest against social security reforms demanded as part of Greece's third international bailout, leaving flights cancelled, schools closed and news broadcasts off the air.

Air traffic controllers joined the strike, shutting down Athens's international airport and forcing airlines to cancel flights.

State-run hospitals and the ambulance service are operating with emergency staff only while engineers, lawyers and journalists are also on strike.

Demonstrations are planned in Athens later.

The reforms, which would see social security contributions hiked to what critics say are unsustainable levels, have led to widespread opposition in a country reeling from six years of financial crisis that has slashed incomes and spiked unemployment to around 25%.