Attempts to win over MP defectors intensified in Sri Lanka's constitutional crisis on Saturday amid growing pressure to let the suspended parliament hold a vote on the two rivals who each claim to be prime minister.

Ousted premier Ranil Wickremesinghe has refused to accept his surprise sacking by President Maithripala Sirisena, who named former strongman president Mahinda Rajapakse in his place.

Rajapakse has eaten into Wickremesinghe's majority amid warnings from pro-democracy and anti-corruption groups about the tactics being used.

A member of Wickremesinghe's United National Party (UNP), S.B. Nawinna, defected on Friday night and was rewarded with the cultural affairs portfolio in Rajapakse's government.

A deputy from a Tamil party supporting Wickremesinghe also switched sides and was made a deputy minister.

“We are expecting a few more defections on Saturday,” a source close to President Sirisena said.

Wickremesinghe has remained in the official prime minister's residence since his sacking on October 26.

He claims to have majority support in the 225-member parliament but is still seeking to woo MPs from the Sirisena-Rajapakse camp, his aides said.

According to latest counts, Wickremesinghe has 103 MPs while Rajapakse and Sirisena together have 100. Most of the 22 remaining MPs are expected to back Wickremesinghe, observers said.

Huge amounts are reportedly being offered to defectors. A UNP stalwart, Range Bandara, said this week he was offered $2.8 million to cross over and support Rajapakse.

A pro-democracy movement urged MPs not to sell their votes and undermine the will of the people.

“We appeal to you not to allow parliament to be put up for sale. You have a responsibility to prevent Sri Lanka from being plunged further into a moral and ethical political abyss,” the Movement for Democracy said.

The Transparency International anti-graft watchdog highlighted the country's Bribery Act which made accepting and offering inducements a jailable offence.

The president suspended parliament for 20 days in a move to forestall a parliamentary vote that would have gone against his choice for prime minister.

But parliament speaker Karu Jayasuriya has called a meeting for next Wednesday. The president has ordered parliament closed until November 16.

“We believe that the proroguing of parliament is to gain time to buy over sufficient cross overs,” the Movement for Democracy said.

It expressed alarm at ministerial positions being given for changing political loyalties.

“We demand a stop to this culture of buying and selling votes,” the group said.

The minority Sri Lanka Muslim Congress (SLMC) party, which has seven lawmakers, said its members had also rejected offers to join the Sirisena-Rajapakse camp.

SLMC leader Rauff Hakeem said the political horse-trading made an early parliament meeting even more important.

Meanwhile, United Nations (UN) Secretary General Antonio Guterres joined international calls for Sirisena to end the suspension of parliament.

The UN chief told Sirisena he was following the crisis in Sri Lanka “with concern,” and “urged the president to revert to parliamentary procedures and allow the parliament to vote as soon as possible,” a UN statement said.