Sri Lankan President Mahinda Rajapaksa's main challenger in the upcoming election will be his health minister and No. 2 of his political party.

Maithripala Sirisena's defection from the government is the biggest political setback for Rajapaksa, who still enjoys considerable support after leading a military campaign to end a 25-year civil war against Tamil Tiger rebels.

Sirisena, who left the government with four other ministers, said Friday he has been chosen as the combined opposition's presidential candidate. No date for the polls has been set yet but they're likely in the first week of January.

Sisisena accused Rajapaksa's administration of concentrating all power in his family and taking the country toward authoritarianism.