Greece's new prime minister has been sworn in after forming a surprise anti-bailout alliance with a small right-wing nationalist party.

Alexis Tsipras broke with tradition and took a secular oath rather than the Greek Orthodox religious ceremony with which prime ministers are usually sworn in.

The 40-year-old, who drew remarks from some observers for not wearing a tie to the ceremony, becomes the youngest man to hold the post in150 years.

His Syriza party gained the key backing of Independent Greeks after Sunday's elections, paving the way for a coalition government.

Tsipras won the vote but fell short of the majority needed to govern alone.

"From this moment, the country has a government. Independent Greeks give a vote of confidence to Prime Minister Alexis Tsipras," Panos Kammenos, leader of the Independent Greeks, said.

Kemmenos did not clarify whether he would join a coalition with Tsipras or give support to a minority government.

The two parties have different ideologies and their coalition comes a surprise, which nonetheless boosted stock markets across Europe that had fallen on news of the uncertain election results.  Stocks had fallen as much as 4 percent in Athens on Monday morning. 

Tsipras has promised to renegotiate Greece's massive bailout agreements, but has vowed not to take any unilateral action against lenders from other eurozone countries. 

Concerns 

With 99.8 percent of the vote counted, Syriza had 149 seats in the 300-member parliament with 36.3 percent of the vote. The ruling conservative coalition was on 27.8 percent, and the extreme right Golden Dawn party in third place with 6.28 percent. 

Tsipras' choice to negotiate with the Independent Greeks - a party aligned in Europe with the UK Independence Party - rather than the centrist Potami caused concern that he could take a tough line in negotiations with rescue lenders, reported the Associated Press news agency. 

Syriza's financial planning official, Giorgos Stathakis, confirmed on Monday that the new government had no plans to meet with negotiators from the "troika", a reference to the European Central Bank, the European Commission and the International Monetary Fund. 

The government would instead seek talks directly with governments, Stathakis said. 

Greek voters swung to the once-marginal left-wing party after five years of punishing austerity measures demanded under $268bn bailout deals drove hundreds of thousands of people out of work and left nearly a third of the country without state health insurance.

Source: Agencies