Three candidates have been nominated to replace former Sri Lankan president Gotabaya Rajapaksa, who fled the country and resigned last week over the island's worsening economic crisis.
Sri Lanka's main opposition leader Sajith Premadasa has withdrawn from the country's presidential election in favour of a ruling party dissident.
Premadasa announced the move on Twitter minutes before nominations formally opened on Tuesday.
He said "for the greater good of my country that I love and the people I cherish" his party will support Dullas Alahapperuma, a senior ruling party lawmaker, to replace Gotabaya Rajapaksa who resigned last week.
His party and "our alliance and our opposition partners will work hard towards making" Dullas Alahapperuma the winner, Premadasa added.
Alahapperuma is up against six-time Prime Minister Ranil Wickremesinghe — who took over as acting president — and leftist leader Anura Dissanayake in a secret ballot on Wednesday.
The winner will take charge of a bankrupt country that is in talks with the IMF for a bailout as its 22 million people endure severe shortages of food, fuel and medicines.
Worst economic crisis since independence
Hit hard by the pandemic and tax cuts by the Rajapaksa government, Sri Lanka is in the midst of its worst economic crisis since it won independence from Great Britain in 1948.
Inflation of over 50 percent and shortages of food, fuel and medicines have brought thousands onto the streets in months of protests that culminated in Rajapaksa's ousting.
Sri Lankan students and other groups planned to protest on Tuesday against Wickremesinghe's bid for president.
The veteran politician is despised by the protesters as an ally of the Rajapaksa clan, four brothers who have dominated the island's politics for years.
Protesters accuse Rajapaksa and his powerful political family of siphoning money from government coffers and of hastening the country’s collapse by mismanaging the economy.
The family has denied the corruption allegations, but Rajapaksa acknowledged that some of his policies contributed to Sri Lanka's meltdown.