New Sri Lankan President Maithripala Sirisena has begun assembling a cabinet on his first day in office as he looks to deliver on pledges to improve the country's diplomatic standing and implement democratic reforms.

Sirisena, who surprisingly defeated veteran strongman Mahinda Rajapaksa in Thursday's election, was trying to form a "national unity" cabinet that would include members from a cross section of political parties in parliament, an aide said on Saturday.

"The main task is to choose a cabinet and the work is already under way," Nishantha Warnasinghe told the AFP news agency.

Sirisena, a one-time ally of Rajapaksa, had offered a 100-day programme to carry out urgent political and economic reforms, including moves to cut back on the powers of the president which Rajapaksa had handed to himself during a decade in office.

Sirisena was elected with 51.28 percent of the votes, to the former leader's 47.58 percent. It was a remarkable setback for a leader who had appeared certain of victory when he called snap polls in November.

Shortly after being sworn in, Sirisena appointed as new prime minister the parliamentary opposition leader Ranil Wickremesinghe, who is expected to wield considerable power.

Wickremesinghe is also seen as having significantly better relations with the West and neighbouring India.

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Rajapaksa fell out with the West over human rights and allegations of war crimes at the end of a drawn-out conflict with Tamil separatists in 2009.  

Several leaders, including the Indian and Canadian prime ministers, boycotted a Commonwealth summit hosted by Rajapaksa in November 2013 over his refusal to allow an international investigation into claims of large-scale killings at the end of the war.

India's Prime Minister Narendra Modi and British leader David Cameron were among the first to congratulate Sirisena.

In an address to the nation soon after being sworn in on Friday, Sirisena promised to mend Sri Lanka's ties with the international community.

"We will have a foreign policy that will mend our ties with the international community and all international organisations in order that we derive maximum benefit for our people," he said.

"We will work with friendship and brotherhood and cooperation with all states."

Source: Agencies