RCOLOMBO (AFP) - Sri Lanka's new government on Sunday accused toppled strongman Mahinda Rajapakse of having tried to stage a coup to cling to power after losing last week's presidential election.

Rajapakse, South Asia's longest-serving leader before losing last Thursday's polls, had been widely praised for conceding defeat to Maithripala Sirisena before the final results had been announced.

But a top aide to Sirisena told reporters that 69-year-old Rajapakse had in fact tried to persuade the island's army and police chiefs to help him stay in office with the use of force.

"People think it was a peaceful transition. It was anything but," Mangala Samaraweera, who is expected to be announced as Sirisena's foreign minister, told a press conference in Colombo.

"The first thing the new cabinet will investigate is the coup and conspiracy by president Rajapakse.

"He stepped down only when the army chief and the police Inspector General (N.K. Illangakoon) refused to go along with him." Illangakoon was "very vocal and did not want to be a party to this coup" while army chief Daya Ratnayake also refused to deploy troops for Rajapakse to seize power, said Samaraweera.

The state attorney general's department also warned that there would be "dangerous consequences".

US Secretary of State John Kerry and even Sirisena himself had thanked Rajapakse for quitting in the early hours of Friday, after his defeat in an election he had seemed certain to win when he called it in November.

Samaraweera said diplomatic pressure had also been brought to bear on Rajapakse, who came in for widespread criticism during his decade in office over his administration's human rights record.

"Some world leaders also spoke with president Rajapakse and prevailed on him to ensure a peaceful transition," Samaraweera said. "I don't know who had spoken (to him), but we know some leaders did talk to him." Samaraweera said it was important for the new administration to disclose what had happened while results were being released, and an independent probe would be a priority of the new cabinet.

Sirisena is expected to deliver an address to the nation later Sunday from the hill station of Kandy, buoyed by securing a parliamentary majority after engineering a spate of defections from his predecessor.

Another top lieutenant said Sirisena had already received the backing of more than 40 lawmakers who were previously loyal to Rajapakse, virtually assuring approval for his programme of radical constitutional reforms.

"We now have more than we need in parliament," Rajitha Senaratne told AFP.

"We can have our legislative programme approved without any difficulty whatsoever." Sirisena previously had the backing of 89 lawmakers and needed another 24 to secure a simple majority in the 225-member house.

The new leader, who is himself a defector from Rajapakse's party, has already pledged to reverse many of the constitutional changes made by his predecessor which gave huge powers to the president.

Sirisena wants to establish independent commissions to run the police, the public service and the judiciary and to transfer many of his executive powers to parliament.

Even Rajapakse's Sri Lankan Freedom Party has said it will support Sirisena's constitutional reforms, making their enactment a formality.

Sirisena quit Rajapakse's cabinet in November to emerge as an opposition unity candidate in the January 8 polls, triggering the biggest defection of lawmakers from a government since independence from Britain in 1948.

In his speech in Kandy, Sirisena is expected to spell out his reform plan in detail and call for a normalising of relations with the European Union as well as other Western nations and neighbouring India.

Rajapakse had alienated many of his fellow leaders by refusing to allow an international probe into allegations of mass civilian casualties in the brutal finale to Sri Lanka's 37-year Tamil separatist war in 2009.

While Rajapakse still retains significant support from the majority Sinhalese community, anger had been growing over rising levels of corruption and his authoritarian style.

Rajapakse had installed relatives in some of the most sensitive posts, including his younger brother Gotabhaya who was often accused of meddling in his officially neutral role as defence secretary.