The 225-seat parliament will vote to choose new leader, who will have to address country's economic and political collapse amid protests, with Ranil Wickremesinghe seen as frontrunner.
Sri Lanka's parliament will be voting for a president to replace Gotabaya Rajapaksa, who fled abroad last week after his palace was stormed by angry protesters now bracing for a crackdown from his likely successor.
The winner of the three-way contest to succeed him will take charge of a bankrupt nation that is in bailout talks with the IMF, with its 22 million people enduring severe shortages of food, fuel and medicine.
Analysts say the frontrunner is Ranil Wickremesinghe, a six-time former prime minister who became acting president after his predecessor resigned, but is despised by the protesters who see him as a Rajapaksa ally.
Rajapaksa's departure wounds a once-powerful ruling clan that has dominated Sri Lankan politics for most of the past two decades after his brothers also quit their posts as premier and finance minister earlier this year.
Wickremesinghe, 73, has the backing of the Rajapaksas' SLPP, the largest bloc in the 225-member parliament, for Wednesday's secret ballot.
'Law and order candidate'
As acting president, Wickremesinghe has extended a state of emergency that gives police and security forces sweeping powers, and last week he ordered troops to evict protesters from state buildings they had occupied.
An opposition MP said Wickremesinghe's hardline stance against demonstrators was going down well with MPs who had been at the receiving end of mob violence, and most SLPP legislators would side with him.
"Ranil is emerging as the law and order candidate," Tamil MP Dharmalingam Sithadthan told the AFP news agency.
Political analyst Kusal Perera agreed Wickremesinghe had a "slight advantage", despite his own party securing just one seat at the August 2020 elections.
"Ranil has regained the acceptance of the urban middle classes by restoring some of the supplies like gas and he has already cleared government buildings showing his firmness," Perera said.
Observers believe that Wickremesinghe will crack down hard if he wins and the demonstrators – who have also been demanding his resignation, accusing him of protecting the Rajapaksas' interests – take to the streets.
Protesters came out on the streets on Tuesday against Wickremesinghe's candidacy.
Outside Colombo's main railway station, a group of several hundred protesters shouted slogans against the Rajapaksa family and Wickremesinghe, who they see as the family's candidate.
"We are protesting again Ranil. He is a corrupted man," said Duminda Nagamuwa, one of the protest organisers. "If Ranil comes we cannot have stability."
His main opponent in the vote will be SLPP dissident and former education minister Dullas Alahapperuma, a former journalist who is being supported by the opposition.
Alahapperuma pledged this week to form "an actual consensual government for the first time in our history".
If he wins, the 63-year-old is expected to name opposition leader Sajith Premadasa as his prime minister. Premadasa's late father Ranasinghe ruled the country with an iron fist in the 1980s when Alahapperuma was a rights campaigner.
The third candidate is Anura Dissanayake, 53, leader of the leftist People's Liberation Front, whose coalition has three parliamentary seats.
Candidates need more than half the vote to be elected. If no one crosses the threshold on first preferences, the candidate with the lowest support will be eliminated and their votes distributed according to second preferences.
The new leader will be in office for the balance of Rajapaksa's term, which runs until November 2024.
If Wickremesinghe is confirmed in the post, he is expected to name public administration minister Dinesh Gunawardena, 73, his schoolmate and a strong Rajapaksa loyalist, as the new premier.