Greek police have fired tear gas to disperse anarchists throwing petrol bombs near parliament in Athens.
Dozens were arrested during the one-day strike against planned spending cuts of 11.5bn euros ($15bn; £9bn).
It was the first union-led action since a conservative-led coalition came to power in June.
The savings are a pre-condition to Greece receiving its next tranche of bailout funds, without which the country could face bankruptcy in weeks.
Greece needs the next 31bn-euro instalment of its international bailout, but with record unemployment and a third of Greeks pushed below the poverty line, there is strong resistance to further cuts.
The government of conservative Prime Minister Antonis Samaras is proposing to save money by slashing pensions and raising the retirement age to 67.
But it has also urged the troika representing Greece’s lenders – the European Commission, the European Central Bank (ECB), and the International Monetary Fund (IMF) – to give it an extra two years to push through the austerity programme.
On Tuesday Greek Finance Minister Yannis Stournaras put a price on that delay for the first time – saying it would in effect cost as much as 15bn euros.
The BBC’s Mark Lowen in Athens describes the scene of clashes between police and some protesters
The Greek protest follows a series of demonstrations in Spain and Portugal, which are also facing stringent austerity measures.
- On Tuesday, 38 people were arrested and 64 injured in demonstrations in the Spanish capital, Madrid, where police fired rubber bullets outside the parliament building – two days before the government is due to announce further austerity measures. The Bank of Spain says the country’s recession is deepening at a “significant pace”
- Portugal is holding an extraordinary cabinet meeting after big protests forced the government to withdraw a plan to raise employees’ tax rates
An estimated 50,000 people joined Wednesday’s protests, including doctors, teachers, tax workers, ferry operators and air traffic controllers.
Greece is caught between rage and despair. With unemployment at a record high of 24% and a third of society under the poverty line, Greeks feel the bailout medicine is not working.
And yet if Greece deviates from the path of austerity reform, its international lifeline – and bailouts to the tune of 240bn euros – would be turned off.
If that were to happen, Greece could face the possibility of bankruptcy, default and exit from the euro in a matter of months.
Another challenge facing PM Antonis Samaras is that he is leading a coalition government where cracks are beginning to emerge about the extent of these austerity measures.
If political instability adds to the financial woes, Greece will be in extremely troubled waters.
Banks and historic sites in Athens remained shut, with many shopkeepers expected to close up early so they could attend demonstrations.
Schools and government services also closed down, though buses were still running, reportedly to help ferry people to the protests.
“We can’t take it anymore – we are bleeding. We can’t raise our children like this,” Dina Kokou, a teacher, told Reuters news agency.
“We won’t submit to the troika!” and “EU, IMF out!”, “People, fight, they’re drinking your blood,” protesters chanted.
A march past parliament turned violent as anarchists wearing black balaclavas and carrying sticks threw petrol bombs and broken bits of concrete at riot police on Syntagma Square.
Images showed a policeman on fire as the bombs exploded.
The strike was called by the country’s two biggest unions, which between them represent half the workforce.
A survey conducted by the MRB polling agency last week found that more than 90% of Greeks believed the planned cuts were unfair and a burden on the poor.
Greece was given a 110bn-euro bailout package in May 2010 and a further 130bn euros in October 2011, backed by the IMF and the other 16 euro nations.
That money is paid in instalments, but correspondents say the lenders are reluctant to stump up the latest slice, as they feel Greece has not made enough effort to meet its deficit-reduction targets.
Greece needs the new money to make repayments on its debt burden. A default could result in the country leaving the euro.