Colombo: Sri Lanka's ruling party today alleged that a foreign governments-backed “systematic campaign” aimed at toppling President Mahinda Rajapaksa who faces an unexpected challenge in next month's polls is being run, but said India is not part of the conspiracy.

“There is a systematic campaign to destabilise Sri Lanka,” Minister of Plantations Mahinda Samarasinghe told reporters.

“I do not want to pinpoint any country. India is not one of those. We have close and strong ties with India,” he said.

Samarasinghe said certain international governments were backing the challenge mounted by the common opposition candidate Maithripala Sirisena against incumbent Rajapaksa.

Sirisena, who was a senior minister in the ruling coalition, defected to the opposition camp along with several other senior leaders of the ruling United People's Freedom Alliance coalition .

“I thank the UNP (United National Party) for choosing me as the common opposition candidate,” Sirisena had said while defecting to the opposition last month.

Sirisena's defection was the biggest political setback for Rajapaksa, who enjoys popularity among Sinhalese majority for leading the Sri Lankan forces to victory after a nearly three-decade-long war against the Tamil rebels.  

But his critics say Rajapaksa has become increasingly authoritarian.

The opposition have announced a plan for democratic reforms calling Rajapaksa administration authoritarian and Rajapaksa family-centric.

The opposition unity candidate has pledged to abolish the system of executive presidency by forming a national unity government.

Rajapaksa is seeking a record-third term after announcing snap polls set for January.

The presidential election will take place on January 8 as Rajapaksa called the election two years ahead of schedule in an apparent attempt to seek fresh mandate before his party's popularity tumbles further, after dropping over 21 per cent in September local elections.