Mahinda Rajapaksa, who is seeking third term as president, faces coalition of parties rallying behind former ally

Voters in Sri Lanka will go to the polls on Thursday in one of the tightest and most significant presidential elections in the island nation for decades.

Mahinda Rajapaksa, already the longest-serving ruler in the region, will seek a third term that would allow him to consolidate what critics say is an increasingly authoritarian and dynastic rule.

But the 69-year-old politician’s decision to call early elections in the hope of an easy victory over a fragmented opposition now looks questionable with a broad coalition of parties rallying behind his former associate turned rival, Maithripala Sirisena.

“It is very unpredictable. It could be very, very close,” said Alan Keenan, an expert with the International Crisis Group.

Few observers expected the poll – which will be won by whoever wins more than 50% of votes cast – to be a genuine contest. Elected first in 2005, Rajapaksa won a landslide in 2010 after bringing Sri Lanka’s 26-year-old civil war to a successful, if bloody, conclusion.

Capitalising on a wave of support from the country’s largely Buddhist Sinhalese majority and widespread expectation of a post-conflict economic boom, Rajapaksa then successfully amended the constitution to allow unlimited terms and concentrated power on the office of president.

Recent local poll results have indicated diminishing support in his core Sinhalese rural support base, a consequence of stubborn unemployment levels and inflation as well as allegations of corruption and nepotism. Three of Rajapaksa’s brothers hold senior official posts. It also seems that his 28-year-old son, already an MP, is being groomed as a successor.

The veteran politician was taken by surprise by the candidacy of former health minister, Sirisena, 63, who walked out of the government a day after polls were called. The soft-spoken farmer turned politician with a reputation as an honest and successful administrator, appeals to the same core conservative Sinhalese rural vote as Rajapaksa but is also seen as the least worst option by Tamils, about 15% of Sri Lanka’s 21 million inhabitants.

Grievances for Tamils include the continuing heavy presence of the Sri Lankan army in northern areas run by the violent separatist Liberation Tigers of Tamil Eelam (LTTE) during the civil war, and a lack of local political autonomy.

“The ordinary [Tamil] voter is just sick of 10 years of a regime which has made no attempt at reconciliation or demilitarisation. They are voting for Sirisena by default,” said Ahilan Kadirgamar, a political economist based in Jaffna, the northern Tamil-dominated town.

Rajapaksa banked on roads, railways and other infrastructure projects to win over largely Hindu Tamils, especially in the conflict-battered north and east, rather than political concessions. “Sirisena has said nothing about devolution or a political settlement but a lot of people believe a change of government might open up some space,” said Kadirgamar.

The biggest Tamil political grouping has endorsed Sirisena’s candidacy. Muslim parties concerned by rising violence from a range of hardline Buddhist groups which have emerged in recent years have also joined the opposition and the ruling party’s coalition has lost more than 20 MPs.

Rajiva Wijesinha, a parliamentarian and former academic, defected in November. “Rajapaksa did a great job getting rid of the [Tamil] Tigers but should have moved on reconciliation and reform,” he said. “He just didn’t seem interested.”

The president appeared increasingly isolated, even blaming the rising Buddhist extremism in Sri Lanka on sponsorship of hardline groups by foreign powers who were set on weakening him by undercutting his popularity with Muslims, Wijesinha told the Guardian.

A former rugby player with a folksy avuncular style, Rajapaksa has been a controversial figure in many western nations. The UN has launched an investigation into allegations of human rights abuses committed by all sides, including the Sri Lankan military, during the last phases of the civil war.

A Commonwealth heads of government meeting in Colombo, the commercial and cultural capital of Sri Lanka, was last year boycotted by India, Canada and Mauritius. David Cameron attended but travelled to the Tamil-dominated north and said he had raised concerns over human rights with his host.

Navi Pillay, while UN’s high commissioner for human rights, expressed fears of growing authoritarianism in Sri Lanka, where in recent years hundreds of journalists, rights campaigners and political opponents have been intimidated and some have disappeared. In one incident last month, severed dogs’ heads were nailed to the houses of Brito Fernando, who runs the Right to Life human rights centre in Colombo, and a fellow campaigner. The men had organised a protest against “dynastic rule”.

“It was done by someone supported by the government or intelligence services,” Fernando said. “I am not afraid. I am worried for my children and my wife but I am not going to stop my campaign.”

Observers say they have recorded at least 237 “major incidents” during the campaigning period, including cases of assaults, intimidation or damage to property. Three supporters of Sirisena were injured by unidentified gunmen on Monday.

“Violence is on the rise and it seems to be a well calculated strategy to prevent the opposition campaign,” Keerthi Tennakoon, executive director of the Campaign for Free and Fair Election, told Reuters.

Sri Lankan police have said they will deploy elite commandos among the force of 65,000 who will guard polling centres.

Opposition leaders have also accused the government of planning to use the military to block people from voting in several regions across the country and using state television to broadcast propaganda. Government officials deny the charges.

“The big question is what the turnout will be,” said Kadirgamar, the Jaffna-based analyst.

In an attempt to ensure support, Rajapaksa, south Asia’s longest serving leader, has cut fuel prices, water and electricity tariffs and given 1.6 million public servants pay increases.

In successive campaign speeches, he has tried to strike a less divisive note, pledging to “protect everyone in this country … Sinhala, Tamil or Muslim” while refusing to compromise on national security.

Observers say coming months are likely to be chaotic and unstable, whatever the result of the vote. “This could be the last chance for Sri Lankan democracy for quite some time … [but] things will be messy even in the best case [scenario],” said Keenan, of the International Crisis Group.