European politicians at opposite ends of the spectrum celebrated the election victory of Greece's anti-austerity Syriza party on Monday, but Alexis Tsipras' post-communist triumph is forcing mainstream leftists to reconsider their position.

Tsipras has promised to reverse five years of "humiliation and suffering" caused by severe spending cuts — the price of a €240 billion bailout from international creditors.

His movement, which is now building a coalition with the right-wing Independent Greeks, will reject fiscal austerity and demand the country's foreign debt be partially written off.