Greece's main opposition party, the radical left coalition SYRIZA, formally merged into a unified party and Alexis Tsipras was elected president with 74 percent of votes on Sunday evening, at the end of the founding congress in Athens.

"As of Monday united in our new party, stronger than ever, we embark on a great and victorious course to put an end to social collapse and rebuild Greece," Tsipras said after the election of SYRIZA's new 200-member central committee.

Earlier, the more than 3,000 delegates approved the party's new charter which foresees the dissolution of the one dozen factions which had joined forces over the past few years around the anti-bailout rhetoric, winning over several disappointed austerity-hit voters.

In last year's general elections, as Greek citizens were split between the tough dilemma of backing parties which supported the painful bailout agreements with international lenders versus parties which held critical stance amidst record high unemployment and deep recession, SYRIZA ranked second for first time in its history.

The coalition garnered 27 percent of votes. In the 2009 general elections, just before the outbreak of the Greek debt crisis, SYRIZA had won about 4.6 percent of votes.

SYRIZA's new political proclamation which was ratified at the end of the four-day congress includes a call for "further write down of part of Greece's debt load after negotiations," rather than an "outright write-down of the entire debt," as suggested by some factions.

The vast majority of delegates also rejected a proposal for the party to leave open the option of Greece leaving the euro and returning to the drachma.

In a signal that the party moves further to the political centre in a bid to win more support, as analysts noted, the new SYRIZA does not call for the nationalization of banks and strategic sectors of the national economy either.

Addressing the congress, Tsipras called on "all democrats" to join SYRIZA and "change Greece."

With the coalition's transformation into a single party, SYRIZA is now eligible in future elections to receive the 50 "bonus" seats given to the top polling party under Greece's electoral legislation.