Seguro succeeds former prime minister Jose Socrates, who resigned as party chief after the Socialists, in power since 2005, were ousted by the centre-right Social Democrat party (PSD) in the June 5 elections.

The new Socialist leader received 23,903 votes in Saturday’s poll against 11,257 (32 percent) for his challenger, legislator Francisco Assis, according near final official results.

The election outcome will have to be endorsed at the party’s next congress scheduled for next September.

Speaking to reporters late Saturday, Seguro said a "new political cycle also means a new way of conducting politics for people and with people."

He also pledged to lead a "firm, responsible, constructive and loyal opposition."

PSD leader Pedro Passos Coelho, 46, was named as prime minister last month with the task of implementing a drastic austerity package imposed by the EU and International Monetary Fund.

The programme involves spending cuts and economic reforms Lisbon agreed in May in exchange for a 78-billion-euro ($110-billion) bailout.

The PSD signed a coalition deal with a smaller right-wing party, pledging to transform the country and win the confidence of the markets.

Portugal owed 160 billion euros at the end of 2010 and its deficit for the year was 9.1 percent of GDP, more than triple the rate allowed among EU members.