Colombo: 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.

The election is taking place, nearly seven months after homegrown radicals pledging loyalty to the Islamic State terror group detonated suicide bombs at three churches and three posh hotels, killing 269 people, seriously hitting the tourism industry, one of the main forex earning sectors of the country.

Prime Minister Ranil Wickremesinghe has urged the people to vote peacefully and maintain law and order to ensure a free and fair election.

Police said that at least 84 people have been arrested for election law violations since the day the election was declared.

Local and foreign election observers are on duty with European Union and Commonwealth observers manning selected areas.

We have mobilized over 4000 stationary and mobile monitors," Rohana Hettiacrachchi of the local monitoring group, Free and Fair Elections (PAFFREL) said.

We have around 3000 monitors at selected polling stations , Manjula Gajanayake of Centre for Monitoring Election Violence (CMEV) said.

The first results are expected after mid night today, officials said.

Outgoing President Sirisena in a statement said he has created the "free environment" to hold the elections during the last four years of his rule and expressed hope that people would turn out in large numbers at polling stations to vote.

In the election, Premadasa, the ruling United National party (UNP) candidate, banks on his 'man of the commoner' image - a legacy of his father Ranasinghe Premadasa, the country's president between 1989 and 1993 until the LTTE assassinated him in 1993.

Premadasa senior was considered as the "man of the poor". His welfare schemes and his low cost housing programmes had endeared him to the masses.

If Premadasa senior is still remembered and loved for his commoner's touch, the dark side of his authoritarian rule still lingers. "We still remember his (Premadasa senior) gory era when we saw piles of bodies of youth burning on the road sides on tyre pyres," Mahinda Rajapaksa, the former President, said at campaign rallies in support of his brother Gotabhaya.

"No one wants to return to the Premadasa terror era," he said.

Premadasa senior led a relentless crackdown on the Marxist JVP uprising between 1987-90.

The Rajapaska senior's legacy of ending the Tamil separatist war has made him the darling of the Sinhala Buddhist majority.

His younger brother Gotabhaya was his top defence ministry official who supervised the daily military operations against the LTTE. While doing so he acquired a reputation as a ruthlessly efficient administrator.

A cleaner capital city of Colombo with spruced up buildings, paved walkways became a symbol of Gotabhaya's efficiency.

His downside was attacking media personnel and crushing dissent under his brother's presidency which culminated in Sri Lanka being subject to UN Human Rights Council resolutions.

Gotabhaya, from Sri Lanka People's Front, to his advantage has the image a man who is most trusted to safeguard national security after the Easter Sunday bombings on April 21 by the Jihadi group which killed over 270 people.

"We want him to tackle the threat of Muslim militancy and ensure national security," Shalani Perera, a first time voter said.

Dissanayake banks on blunders made by the major parties over the last 70 years since Sri Lanka gained independence.

"Both have shown they are incapable of handling any of your issues, they are both corrupt," Dissanayake told campaign rallies.

The NPP who is dominated by Dissanayake's Marxist JVP has the unsavory legacy of two bloody rebellions, in 1971 and 1987-90.

Since the late 90s they have campaigned extensively on good governance and anti-corruption platforms. They stood well against last year's constitutional coup when President Sirisena dismissed the government unconstitutionally.

Sirisena sacked Prime Minister Wickremesinghe and installed Mahinda Rajapaksa to replace him, plunging the country into constitutional turmoil.

After the Supreme Court's intervention, Wickremesinghe was reinstated.