Police fired tear gas and clashed with demonstrators in Athens after some 50,000 people finished a peaceful march against cutbacks intended to fix the country's debt crisis.
The violence lasted about 30 minutes, when scores of youths hurled rocks, red paint and plastic bottles near parliament. Police said at least two people were detained, while several storefronts were vandalized.
Windows were smashed at the Finance Ministry's General Accounting Office, which has been accused by the European Union of slipshod statistics-keeping that made the financial crisis worse.
The day's protests were otherwise peaceful. Labor unions organized the protest march amid a 24-hour general strike that grounded flights, shut schools and crippled public services, in a show of strength against the government.
The walkout comes as Greece is considering tougher austerity measures to ward off a financial crisis that has undermined the euro and raised fears that financial market contagion will spread to other weak economies such as Portugal, Spain and Italy.
The European Union has issued a vague promise to support Greece, which has some 53 billion euros ($71.9 billion) in debt coming due this year, but the government of Prime Minister George Papandreou has pressed for more specific guarantees to shore up market confidence.
The government says it won't need a bailout and will stick to its plans to make sharp cutbacks.
There was disruption elsewhere in Europe by workers unsettled by the threat to their jobs from the slow economy and government plans to cut back. In France, a strike by air traffic controllers disrupted flights for a second day.
And in Spain, tens of thousands of demonstrators on Tuesday rallied in several cities to protest a government proposal to raise the retirement age by two years to 67.
Greece has already imposed broad spending cuts but says it is under pressure from the European Union to cut salaries in the civil service, traditionally looked to as a jobs provider.
Officials from the EU and International Monetary Fund are in Athens to inspect public finances, ahead of a March 16 deadline to show signs of fiscal improvement or face imposed additional austerity measures.
Unions say cutting Greeks' so-called 14th salary — part of annual pay held back as a holiday bonus — for public workers would be taken as "an act of war."
"We're all here for the same reason: the measures the government is taking. They have to listen to us," said musician Dimitris Petridis, who took part in the protest rally, banging a snare drum along with several colleagues to a funereal rhythm.
"The rise in joblessness has really hurt us. The daily wage for working at a nightclub, for many of us, is the same as it was 20 years ago."
The country's two largest umbrella labor groups, the private sector GSEE and public sector ADEDY, fiercely oppose a wave of belt-tightening measures announced over the past weeks to reduce the bloated budget deficit from 12.7 percent of gross domestic product to 8.7 percent this year.
"If all these measures are enforced, unemployment will skyrocket. Our country will enter a massive recession and unemployment will reach a Europe-wide record," GSEE spokesman Stathis Anestis said.
"This will be tragic because it will provoke social (unrest) and clashes."
The country's woes have affected confidence in the euro as a common currency, and hiked the country's borrowing costs.
Greek unemployment hit a five-year high of 10.6 percent in November 2009, up from 9.8 percent in October.
Interest-rate yields on Greek bonds remained high on Wednesday, reflecting market worries of a default. Spreads on government bonds over their German equivalent was at 3.30 percentage points after Fitch ratings agency on Tuesday downgraded ratings for four Greek banks. The spread had been lower after the EU's promise of support.
Market confidence has been undermined by evidence that Greece's statistics agencies have been misreporting the state of its finances for years.
Greece's central bank governor George Provopoulos said in a speech Tuesday that the country's crisis heightened a pressing need for major economic reforms.
"The crisis could present an opportunity to carry out necessary reforms — and not just have a debate about them — given that not implementing these reforms would have a great price."
Wednesday's strike is a crucial test of support for the unions, with polls showing strong public support for the government's austerity plan.
A poll Sunday in the Ethnos newspaper showed some 57.6 percent of Greeks believe measures taken so far are "in the right direction," while 75.8 percent think unions should show restraint until the end of the crisis.
The strike, which ends at midnight Athens time, grounded all flights at Greek airports and left trains and ferries idle. Commuters in Athens were left without most forms of public transport. State-run schools, tax offices and municipalities were all closed, while public hospitals used emergency staff. Journalists also held a 24-hour walkout.
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