A senior member of Sri Lanka's government has announced he is quitting the ruling party to stand as the main opposition's candidate against President Mahinda Rajapaksa in upcoming elections.

"I thank the UNP (United National Party) for choosing me as the common opposition candidate," said Health Minister Maithripala Sirisena, who is also general secretary in Rajapaksa's Sri Lanka Freedom Party.

"I am grateful to the UNP for choosing the general secretary of the SLFP to be their candidate," he added. "We will definitely win."

Sirisena made his announcement at a press conference in Colombo, seated alongside three other ministers and former Sri Lankan president Chandrika Kumaratunga, who is the main patron of the president's party.

'Dictatorship'

Rajapaksa, who came to power in 2005, declared on Thursday that he will seek an unprecedented third term as president - a move that was only made possible after he pushed through changes to the constitution.

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While Rajapaksa remains generally popular with majority Sinhalese voters after overseeing the end of a 37-year war against Tamil separatists in 2009, critics say he has become increasingly authoritarian.

"The country is heading towards a dictatorship," said Sirisena, who also accused the president of nepotism.

"The entire economy and every aspect of society is controlled by one family," he said in reference to the president's brothers who include the speaker of parliament Chamal Rajapaksa.

Although the exact election date has yet to be announced, the vote is widely expected to be held in the second week of January.

Source: Agencies