He said the defection of Maithripala Sirisena, who was his health minister, to the opposition was a conspiracy against him.
"We will not be deterred by (such) conspiracies. We will continue with our work," Rajapaksa said at a gathering in north-central province yesterday.
Rajapaksa, who enjoys support among the Sinhalese majority, claimed he had freed the island from terrorism and given unprecedented development. He said he would continue unabated.
Sirisena, 63, defected to opposition on Friday and announced that he would contest the January 8 polls and challenge Rajapaksa, who is seeking an unprecedented third term.
Sirisena's defection is not the only setback Rajapaksa is facing, another minister, a deputy minister and at least three other lawmakers have left the ruling coalition -- United People's Freedom Alliance.
Rajapaksa, 69, who successfully led the government offensive against the Tamil Tigers and defeated them in 2009, has also been accused of being authoritarian.
Sirisena while defecting alleged that following the military victory against the LTTE in 2009, the Rajapaksa government ventured in the wrong direction.
Sirisena accused the Lankan President of nepotism and corruption and also that he accumulated power in his hands.
Sirisena's defection is the biggest political setback for Rajapaksa, who enjoys popularity among Sinhalese majority. The defections and the unity shown by a previously fragmented opposition have cast doubt over Rajapaksa's re-election prospects.
Earlier this month, the National Heritage Party (JHU) announced it was exiting the government. As the main party of Buddhist monks, its departure could damage Rajapaksa's support in a country where Buddhists comprise nearly 70 per cent of its population of 21.8 million.
The JHU had urged Rajapaksa to set in democratic reforms before calling for elections to seek a third term. Rajapaksa did not respond to their calls.
Sirisena, however, has pledged to introduce political and electoral reforms within 100 days of being elected.