Police have fired tear gas in Greece to disperse anarchists throwing petrol bombs near parliament in Athens.
Dozens were arrested during the one-day strike against planned spending cuts of 11.5 billion euros ($15 billion).
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 31 billion-euro installment 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 15 billion euros.
The Greek protest follows a series of demonstrations in Spain and Portugal, which are also facing stringent austerity measures.
An estimated 50,000 people joined Wednesday’s protests, including doctors, teachers, tax workers, ferry operators and air traffic controllers.
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 110 billion-euro bailout package in May 2010 and a further 130 billion euros in October 2011, backed by the IMF and the other 16 euro nations.
That money is paid in installments, 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.