Industrial action began on Tuesday but has now been ramped up nationally by members of the big trade unions.
Ferry services stopped and buses and trains were limited. Flights were also being hit for several hours.
MPs are set to vote on controversial reforms on Thursday that will cut pensions and end tax breaks.
The left-wing Syriza government needs to make further savings before eurozone finance ministers agree to hand over further loan cash as part of its latest bailout deal.
Despite pouring rain, crowds of protesters joined a rally late in the morning in Klafthmonos Square in the centre of Athens. Further rallies were taking part elsewhere in the capital, including one organised by the communist-affiliated PAME union.
A large number of professions were involved in the strike, including the biggest trade unions, the GSEE and ADEDY:
- Air traffic controllers stopped work for four hours
- No newspapers were published and news websites and TV channels were also badly hit
- Doctors and other hospital workers walked out - and hospitals limited services to emergencies only
- Trains, buses and trams were providing only limited services for some hours of the day
Civil servants, court officials and teachers were all involved. Police and other security forces were expected to join a protest late in the afternoon. The new austerity measures will not come into force until 2019 and 2020 but Prime Minister Alexis Tsipras has to persuade eurozone finance ministers next Monday to provide the next loan instalment of €7.5bn (£6.4bn; $8.2bn).
Greece is facing its next big debt demand in July and has had three bailouts from the EU and IMF so far.
The country has fallen back into recession for the first time since 2012, according to figures that emerged on Tuesday. Gross domestic product (GDP) fell by 0.1% in the first three months of 2017 after shrinking by 1.2% in the final quarter of last year