Tsipras sworn in as Greek prime minister

Syriza leader Alexis Tsipras raises his fist as he greets supporters following victory in the election in Athens on January 25, 2015 (AFP Photo/Aris Messinis) (AFP)

Athens (AFP) - Alexis Tsipras, the 40-year-old leader of anti-austerity party Syriza, was sworn in on Monday as Greek prime minister, the youngest in 150 years.

Tsipras -- wearing a blue jacket and white shirt, characteristically without a tie -- also broke with tradition by taking a civil instead of a religious oath, pledging to "always serve the interests of the Greek people".

Tsipras' party took more than 36 percent of the vote in Sunday's general election, becoming the first elected movement in Europe openly opposed to austerity.

The leftists plan to challenge the multi-billion EU-IMF bailout, which they say has sparked a "humanitarian crisis" in Greece.

They won 149 seats -- two short of the required 151-seat majority in parliament -- but covered the shortfall through an alliance with nationalist party Independent Greeks (ANEL).

The coalition with ANEL has been described as peculiar, as the two parties are mainly united by their opposition to the EU-IMF bailout but share little common ground beyond that.

Renewed fears that Greece could be forced out of the eurozone if it defaults on its debt repayments saw the euro briefly hit an 11-year low against the dollar while Greek stocks also sank early Monday.

Syriza's victory could inspire other anti-austerity parties in Europe, including Spain's Podemos, which has topped several opinion polls and is seeking a majority in the Spanish election due in November.