COLOMBO: Sri Lankan authorities ringed parliament with heavy security on Wednesday as lawmakers prepared to choose between three candidates for president, hoping the new leader could pull the island out of a crippling economic and political crisis.

A win for acting President Ranil Wickremesinghe, one of the main two contenders but opposed by many ordinary Sri Lankans, could lead to more demonstrations by people furious with the ruling elite after months of crippling shortages of fuel, food and medicines, several protesters have said.

"If Ranil comes (into power), we cannot have stability," said Duminda Nagamuwa, who organised protests in Colombo after the nominations were finalised. Other protesters have said the acting president is an ally of the powerful Rajapaksa family, who they blame for the economic crisis.

The other main candidate, ruling party lawmaker Dullas Alahapperuma, is more acceptable to the protesters and the opposition but does not have any top-level governance experience in a country with barely any dollars for imports and desperately in need of an IMF bailout.

The third candidate, Anura Kumara Dissanayaka, the leader of the leftist Janatha Vimukti Peramuna party, commands only three seats in the 225-member parliament and has no realistic chance of winning. 

Hundreds of police, paramilitary and military troops were deployed around the parliament building and an approach road had at least three barricades. Security personnel in speed boats patrolled a lake around the building, and military jeeps and armoured vehicles stood parked within the perimeter.

Wickremesinghe, a six-time prime minister, became acting president last week after then president Gotabaya Rajapaksa fled to Singapore when hundreds of thousands of people came out on the streets against his administration and occupied his official residence and office.

Protesters also burned down Wickremesinghe's private home and stormed his office.

It was not immediately clear how much support 73-year-old Wickremesinghe and 63-year-old Alahapperuma have in parliament.

Wickremesinghe is backed by a section of the ruling party that had a total of 145 seats as of the last parliamentary election in 2020. Alahapperuma has the support of the other section as well as the main opposition party that won 54 seats last time round. Several small parties have promised to support him as well.

Latest numbers are not clear because some lawmakers have become independents.

"Earlier Ranil Wickremesinghe was the front-runner but now the outcome is much more uncertain," said political scientist Jayadeva Uyangoda.

"The balance of parliament power has shifted away from him. The outcome is dependent on how much control the Rajapaksas have ... over their party members."

Sri Lanka's parliament in 1993 unanimously chose D.B. Wijetunga to finish the tenure of assassinated President Ranasinghe Premadasa. This time three candidates are in the fray to complete Rajapaksa's term, scheduled to end in 2024.

"It will be marked as a new experience in the parliamentary history of this country," a statement from the communication chief of parliament said, laying out the procedure.

A candidate receiving more than one-third of the valid votes cast will be declared elected. If no candidate reaches the mark, the one with the lowest number of votes will be eliminated from the competition and preferences of lawmakers taken into account to eventually arrive at a winner.