The Radical Coalition of the Left (Syriza) has become Greece's second-largest party following its dramatic success in Sunday's parliamentary election.
It has campaigned steadfastly against the recovery blueprint for Greece drawn up by the European Union and International Monetary Fund.
Previously Greece's fifth largest party, this loosely-knit coalition of left-wing groups has new influence as the country embarks on a tortuous course to form a new government and complete painful economic reforms.
The party is led by the country's youngest political leader, the brash and self-assured Alexis Tsipras.
The 37-year-old believes Greece is heading in the wrong direction and ought to ditch the international bailout.
His attacks on the unpopular EU-IMF plan, known in Greece as the ''memorandum'', tapped into a deep vein of outrage.
He said the election result represented radical change in both Greece and Europe, and showed that people were not prepared to settle for "barbarous memorandums" and bailouts.
He has promised to freeze payments to creditors and renegotiate measures included in Greece's latest 130bn-euro ($173bn; £110bn) rescue package.
For two years, Mr Tsipras rarely pulled his punches in parliament as he criticised the reforms demanded by the EU and IMF in return for a credit lifeline.
He has been described as a cool, mild-mannered politician who shuns neckties and likes to get around on his motorcycle.
Born four days after the fall of Greece's military dictatorship in July 1974, Mr Tsipras became leader of Syriza in 2008 and was elected to parliament in 2009. He first emerged on the political scene in 2006, when he came third in the Athens mayoral race.
The former communist youth activist has been accused by the socialists of inciting violent protests.
Syriza said in March that it had sued Germany's Bild newspaper for a million euros over "insulting" references made in a recent article on the country's upcoming elections. Syriza said the German tabloid had portrayed Mr Tsipras as a "half-criminal" who "openly supports violent anarchists".
Syriza was formed before the parliamentary election in March 2004, with the Coalition of Movements and Ecology (Synaspismos) as its main member. It won six seats with 3.26% of the vote.
It went on to win 14 parliamentary seats with 5.04% of the vote in the 2009 parliamentary election.
It lists the following parties and organisations as components of Syriza: Synaspismos (Coalition of the Left of Movements and Ecology); AKOA (Renewing Communist Ecological Left); KOE (Communist Organisation of Greece); DEA (Internationalist Workers' Left); Kokkino; Xekinima; Rosa; KEDA (Movement for the Unity in Action of the Left); Energoi Polites (Active Citizens); Ecosocialists Greece; DIKKI (Democratic Social Movement).
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