Police said up to 40,000 people were participating in two separate marches in central Athens. Limited clashes broke out when hooded youths threw fire bombs and stones at riot police, who responded with tear gas. No arrests or injuries were immediately reported.
"We are protesting about (reduced) pensions, emergency taxes, the high cost of life," said retired factory worker Kyriakos Anastassiadis.
Unions are pressing for the renewal of binding collective labor contracts, instead of individual deals that allow employers greater leverage in defining salary levels. They are also asking the government to support the crumbling labor market, where roughly 1,000 jobs have been lost daily since 2010.
"The government wants to further lower our salaries and destroy unions," said unionist Vassilis Epicarithis, an aircraft engineer.
The 24-hour nationwide walkout disrupted domestic flights, kept ferries and long-distance trains idle and crippled public services. It was the first general strike of the year, renewing confrontation between labor groups and the conservative-led government that has pursued punishing austerity policies to cut debt
-- a key condition imposed by international bailout creditors.
State schools and tax offices closed down, public hospitals functioned on emergency staff, court cases were stalled as lawyers walked off the job, and even neighborhood street fruit and vegetable markets were cancelled. Private doctors and dentists also joined the strike.
In Athens, police said about 25,000 people were marching toward Parliament with banners such as "We won't become slaves in the 21st century," in a demonstration organized by the main public and private sector unions. Earlier, some 15,000 members of a Communist Party-affiliated labor union protested peacefully along the same route.
Previous protests have been marred by clashes between riot police and anarchists. Up to 3,000 police officers were on duty for the Athens street rallies.
In the northern city of Thessaloniki, some 17,000 protested peacefully.
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Conservative Prime Minister Antonis Samaras has won praise from bailout lenders for pushing through major cost-cutting measures after forming a three party coalition last June.
But a new round of tax increases this year and a surge in unemployment to 27 percent have angered unions, as Greeks battle a rapid increase in poverty during a sixth year of recession.
In recent weeks, the Samaras government has twice used rare emergency powers to force an end to strikes by workers on ferry services and the Athens subway.
"The Greek people have no tolerance left," Ilias Iliopoulos, general secretary of the civil servants union ADEDY, told the AP in an interview ahead of the strike. "For us, the time has come for a major confrontation with the government ... and policies that are taking our country from bad to worse and leading people to poverty and desperation."
Unions are also angry at a government decision to scrap collective wage agreements across the public sector as part of an overhaul of state pay scales that will usher in further salary cuts.
Unemployment is expected to reach 30 percent this year, while national output will contract a further 4.1 percent, according to a study by a government funded research agency published last week. By the end of the year, the Greek economy is forecast to have shrunk 25 percent since 2008, a year before the crisis started.
Press; By ELENA BECATOROS and NICHOLAS PAPHITIS]
Derek Gatopoulos, Rafael
Komminis and Annita Mordachai contributed to this report.
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