Greek public and private sector workers have gone on strike, grounding flights, shutting schools and halting public transport.
The action is the second nationwide walkout in two weeks in protest against the government's austerity plans, which are aimed at cutting the country's huge debt.
The government plans to save $6.5bn with increases in sales tax, reductions in civil servant wages and a pensions freeze.
The GSEE and Adedy, its public-sector sister union, said that the measures will hurt the poor and worsen the economy.
About half of the nation's five million workers are represented by the unions for the 24-hour strike.
About 1,500 riot policemen are on duty in the centre of Athens, the capital, with Greek strikes often turning violent in the past.