Colombo: Sri Lanka's former wartime defence chief Gotabaya Rajapaksa was set to become president after his main rival conceded defeat on Sunday in an election that came months after bombings by Islamist militants threw the country into turmoil.

Rajapaksa, who oversaw the military defeat of Tamil separatists under his brother and then president Mahinda Rajapaksa 10 years ago, has promised strong leadership to secure the island of 22 million people, the majority of whom are Sinhalese Buddhists.

With half the votes counted from Saturday's election, Rajapaksa led with 50.7 per cent, while his main rival Sajith Premadasa had 43.8 per cent, the election commission said. Rajapaksa's party claimed victory.

Premadasa, a housing minister in the current government that has faced criticism for failing to protect Sri Lankans in the wake of the suicide bombings in April, conceded defeat.