COLOMBO - A senior minister quit Sri Lanka’s ruling party Friday to stand as the main opposition’s candidate in upcoming elections as he accused President Mahinda Rajapakse of turning the country into a dictatorship.

Health Minister Maithripala Sirisena, who is also general secretary in Rajapakse’s Sri Lanka Freedom Party, said he was confident of victory in January’s presidential election when he will stand as the main opposition United National Party’s candidate.

“I thank the UNP for choosing me as the common opposition candidate,” said Sirisena, in what represents a stunning blow to Rajapakse’s authority. “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 - sat alongside three other ministers and former Sri Lankan president Chandrika Kumaratunga, the main patron of the president’s party, in a show of support for the new opposition candidate.

Rajapakse, who came to power in 2005, declared 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.

While Rajapakse 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 63-year-old Sirisena, who also accused the president of nepotism.

“The entire economy and every aspect of society is controlled by one family.”

The president’s brothers include the speaker of parliament Chamal Rajapakse, the economic development minister Basil Rajapakse and the powerful defence secretary Gotabhaya Rajapakse.

His eldest son Namal is a lawmaker and heads the SLFP’s youth wing.

“Corruption is rampant, there is no rule of law,” added Sirisena who promised to reverse the constitutional amendments brought in under Rajapakse which have increased the powers of the president.

“I urge you to support me to scrap the executive presidency. I will scrap this executive within 100 days.”

The contest is taking place against a backdrop of growing international pressure over the Rajapakse administration’s human rights record.

Rajapakse, who is the longest serving leader in South Asia, is struggling to avoid international censure over allegations his troops killed 40,000 Tamil civilians in the bloody finale of the fighting.

The 69-year-old also faces accusations that his administration has silenced dissenting voices, including the media and judiciary after he sacked the chief justice last year.

While the exact polling date will be announced by the chief elections commissioner, an official in the president’s office told AFP Thursday that the vote was expected on either January 7 or 8.

Pro-government astrologers have previously said the first week of January represents the most auspicious window of opportunity.

The government is keen to hold the ballot before a scheduled visit by Pope Francis from January 13 to 15, with the Church warning that parties should not use the trip for political advantage.

Rajapakse came to power after narrowly winning the 2005 election, and increased his majority in 2010 after government troops routed the Tamil Tigers.

However the spectacular military success also sparked war crimes allegations, and Rajapakse’s administration is facing an international probe ordered by the UN Human Rights Council in line with a US-led resolution in March.

His ongoing two-year chairmanship of the Commonwealth has also been overshadowed by the allegations.