The bomb attacks on the house of a ruling party activist in the northern peninsula of Jaffna underlined the bitter nature of the contest between President Mahinda Rajapakse and his main challenger, former army chief Sarath Fonseka, which threatens to bring more instability to the violence-torn nation.
"We have a complaint that two bombs were thrown at the home of Subramaniam Sharma, an organiser for the Sri Lanka Freedom Party," Centre for Monitoring Election Violence (CMEV) spokesman D. M. Dissanayake said.
It was not immediately clear who carried out the attack, he said.
Last May, Rajapakse and Fonseka wiped out Sri Lanka's Tamil Tiger rebels, who had fought for a Tamil homeland since 1972, in a military campaign since dogged by allegations of war crimes.
But the one-time close battlefield allies have turned into enemies after Fonseka, a 59-year-old political novice, decided to challenge his former boss at the ballot box on an anti-corruption platform.
In the acrimonious run-up to the vote, the opposition and government have accused each other of malevolent intentions, raising tensions across the country and the prospect of a contested result.
Fonseka alleged the government intended to unleash violence to intimidate voters and was preparing a coup if it lost. He was accused in turn of working with a militia of army deserters who could disrupt the vote.
In Jaffna, residents reported hearing four explosions before dawn Tuesday, but the monitors could only immediately account for two of them.
"We are not sure from where the blasts originated or who was responsible," CMEV monitor Sunil Jayasekara said by telephone.
At least four political activists were killed and nearly 1,000 election-related incidents of violence were reported to police before the polls, which involve 22 candidates in total.
The house of a key opposition fundraiser was bombed last Friday in Colombo.
There are no reliable opinion polls in the country and political observers say the election is too close to call between Rajapakse and Fonseka, the only two candidates with any realistic chance of winning.
Both camps believe they can claim a majority, but the pro-Fonseka opposition has said it will not accept a result if the 68,000 police and 12,000 soldiers on duty fail to prevent violence.
In the suburbs of Colombo, people had lined up half an hour before the polls opened across the country at 7:00am. Voting was to close at 4:00pm (1030 GMT).
In the northern Tamil area of Vavuniya, people began trickling towards the polling stations once they opened.
The first results were expected to emerge late on Tuesday and a final outcome was anticipated around midday on Wednesday, which has been declared a public holiday.
Fonseka has hardened his rhetoric in recent days, pointing to alleged troop movements, plans to disrupt the media and instructions to the police as evidence that the government will use the army to stay in power if necessary.
"These are the indications of a military coup," he told reporters on Monday in his last pre-poll press conference, surrounded by his technicolour coalition of Marxists, Tamils, Muslims and right-wingers.
"If there is a war, we will face it," he added, threatening street protests and rallies.
Rajapakse, like Fonseka a nationalist from the majority Sinhalese ethnic group, has vowed to ensure the poll goes off peacefully and had called on voters to give him a second mandate to develop the country post-war.
"All necessary instructions have been given by me for the maintenance of law and order," he said in a statement received late on Monday.
Rajapakse's supporters are drawn by the mustachioed 64-year-old's charisma and populist approach and see him as the man who liberated the country from a fight with the Tigers that cost 80,000-100,000 lives, according to the UN.
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