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Sanka Gayashan, Associated Press
Sri Lankan President Mahinda Rajapaksa laughs during his final public rally for the presidential elections in Kesbewa, about 20 kilometers (12 miles) southeast of Colombo, Sri Lanka, Monday, Jan. 5, 2015. A confident Rajapaksa had called the election two years ahead of schedule, hoping to win a third six-year term before voters' memories faded of the defeat of the Tamil Tiger rebels. But an internal revolt now threatens his hold on power with Health Minister Maithripala Sirisena, a close Rajapaksa aide and No. 2 in the president's Freedom Party, defecting and announcing he would run as an opposition candidate in Thursday's election. Poster reads "Leader of the common.”

COLOMBO, Sri Lanka — A monitoring group expressed concerns Tuesday that the military presence in the former war zone of northern Sri Lanka may lower voter turnout in the presidential election Thursday.

Pakiasothy Saravanamuttu from the Centre for Monitoring Election Violence said that concerns have been raised about the vote in the north in particular and about the deployment of security forces that could have "an adverse impact on the voter turnout."

Sri Lanka's military has been accused of intimidating opposition voters in previous elections and campaigning for President Mahinda Rajapaksa.

He has won two previous elections on a wave of popularity for ending a 25-year civil war after militarily defeating the Tamil Tiger rebels, who wanted an ethnic Tamil state in the island's north and east. An ethnic minority in Sri Lanka overall, Tamils are the majority in the former war zone in the north. Both Rajapaksa and opposition candidate Maithripala Sirisena are from Sri Lanka's Sinhalese ethnic majority.

Rajapaksa is facing his biggest election challenge in the revolt led by Sirisena, his former health minister.

Calling the election two years early was seen as an attempt by Rajapaksa to prevent defeat if the poll was held on schedule.

Sirisena defected from Rajapaksa's government a day after the election was announced because he said Rajapaksa was abusing his power and taking the country toward autocracy.

An ultranationalist Buddhist party, the country's largest Muslim political party and its main ethnic Tamil political party are supporting Sirisena. The main opposition United National Party and 26 lawmakers who defected from Rajapaksa's government also support Sirisena.