Gunmen opened fire on a convoy of buses carrying minority Muslim voters in northwest Sri Lanka as Sri Lankans voted Saturday to choose a new President in an election that will decide the future of the country that struggles with security challenges after the Easter Sunday bombings and increasing political polarisation.

It is a close contest between former wartime Defence Secretary Gotabaya Rajapaksa, 70, and the ruling party candidate Sajith Premadasa, 52. Anura Kumara Dissanayake from the National People's Power (NPP) coalition is also a strong candidate.

Polls opened at 7 am local time and would close at 5 pm with some 12,845 polling stations being set up throughout the country for 15.9 million voters, who will choose a successor to President Maithripala Sirisina among a record 35 candidates.

Sirisena, who was elected in 2015, is not seeking a re-election. 

Despite tight security, gunmen attacked a convoy of 100 buses transporting minority Muslim voters in northwest Sri Lanka, hours before polling got under way, the AFP reported.

There were no immediate reports of casualties, but a police official said the attackers had burnt tyres on the road and set up makeshift road blocks to ambush the convoy of more than 100 vehicles  in Tantirimale, 240 kilometres north of Colombo, the report said.

Over 60,000 security personnel have been deployed to provide adequate security for the election, police spokesman Ruwan Gunasekera.