Riot police fired tear gas to disperse the crowds after 50 hooded protesters began hurling Molotov cocktails, bottles and rocks at officers, destroying store fronts, traffic cameras, banks and ATM machines in their path.

Police also clashed with protestors outside the Turkish Consulate after they attempted to damage the entrance. Reports said 25 people were arrested.

Karamanlis is midway through his second four-year term, which would normally end in 2011. However, the main opposition Socialists had threatened to force an early vote in March when parliament is to elect a new president, requiring a two-thirds majority.

By announcing snap elections, Karamanlis officially launched a difficult, month-long election campaign where the ruling conservatives are trailing the main opposition in polls.

A recent survey published in centre-left To Vima newspaper gave the Socialists a 5.7-to-6.7-point lead.

Main opposition Socialist leader George Papandreou called the election announcement by Karamanlis an admission of his failure in dealing with the country's problems.

The conservatives, who currently have a one-seat majority in the 300-member parliament, have been hit by months of scandals and Greece's worst riots in decades.

Their popularity has sunk further after recent wildfires hit the Greek capital last month.