ATHENS, Greece (CNN) -- Greece's opposition Socialist party on Sunday defeated the incumbent center-right government of Prime Minister Kostas Karamanlis, as Socialist leader George Papandreou promised to chart a new course for an economic comeback. Socialist George Papandreou is set to become Greece's next prime minister. "On this course, nothing is going to be easy -- it will take work, hard work," Papandreou said in his victory address. "And I will always be honest with the Greek people so that we may better solve the problems of the state." Sunday's national elections were held two years before originally scheduled. Karamanlis called the elections in response to pressure from Papandreou's Panhellenic Socialist Movement of Greece, which threatened to block the election of a president in February if no general election was held. The Greek constitution requires the two major parties to agree on the election of a president, giving either party an effective veto.
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87% of votes counted; Socialists to get about 160 seats in 300-seat Parliament
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Socialist leader George Papandreou to Greeks:  "I will always be honest"
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During campaign, Papandreou proposed massive stimulus for weak economy
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Sunday's national elections were held two years before originally scheduled
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