Incumbent President Mahinda Rajapakse, who ordered the bloody military offensive that crushed the Tigers, is facing the strongest challenge from Sarath Fonseka, the army chief who led the troops in battle.
Rajapakse urged the authorities on Monday to conduct a free and fair poll and ensure that people could vote without fear of violence.
"The Sri Lanka government calls for a peaceful election, and stands committed to taking whatever steps deemed necessary to ensure the same," his office said in a statement.
About 120,000 public officials will oversee the ballot, in which 14 million people are eligible to choose from 22 candidates.
"We have sent ballot boxes to the provinces and all public servants involved in the conduct of the poll will be at their stations by noon today," a spokesman for the independent Elections Commission said.
The two Sinhalese nationalists fell out after the war, with Fonseka retiring from the military after he was sidelined by Rajapakse and launching his surprise challenge for the presidency.Fonseka's side said Sunday there was evidence of a coup plot by Rajapakse's ruling party to try to hold on to power but the government denied any intention to subvert the electoral process.
Rajapakse called Tuesday's vote after only four years of his six-year mandate to try to harness public acclaim for victory over the Tiger rebels, who controlled one-third of Sri Lanka just 10 years ago.
The conflict between government troops and the Tigers cost between 80,000 and 100,000 lives, according to UN estimates.