The protests also affected banks, state hospitals, post offices and most other public services.
Greek air traffic control walked off the job between noon and 4 p.m. (1000GMT and 1400GMT), halting all traffic at Athens International Airport and other Greek airports.
''There will be no take-offs and landings during these hours,'' Athens airport spokeswoman Marina Papageorgiou said. ''But this protest was known in advance, and the airlines have rescheduled their flights - keeping problems to a minimum.''
State-run carrier Olympic Airways canceled more than 50 domestic and international flights Thursday.
Most of the capital's transport services, including the metro system, were also on strike.
About 6,000 people attended two protest rallies in central Athens organized by Greece's largest labor unions, and demonstrators blockaded the entrance of the development ministry's commerce department. Demonstrations were also held in other Greek cities.
In a brief scuffle, police in Athens fired pepper spray to disperse a small group of students demonstrating in front of parliament, but the rally ended peacefully and no arrests were reported.
Premier Costas Caramanlis, whose conservative party took office a year ago, has promised to speed up privatization, extend store opening hours and loosen labor regulations in an effort to reduce unemployment, stubbornly hovering above 10 percent.
The government, under a deficit warning from the European Union, is hoping to raise euro1.6 billion (US$2.1 billion) from the sale of state holdings, including shares in the Public Power Company and natural gas utility.
But unions argue the reforms will further erode workers' rights, and are calling for government intervention to limit the number of part-time contracts, control prices of basic goods, and stop local businesses moving their factories from Greece to other Balkan countries.
''Workers and pensioners are being deceived,'' Christos Polyzogopoulos, head of the General Confederation of Greek Workers, said in a statement. ''We want real wage hikes and real jobs ... not a return to labor middle ages.''
British tourists Stephen and Gillian Hart, from Lancashire in northern England, were in central Athens when demonstrators waving red flags and chanting ''Resistance is the only way,'' filed past them.
''We've been walking around Athens, we wondered what it was,'' Stephen Hart said.