Sri Lanka's opposition Thursday accused the government of deploying thousands of troops to deter ethnic minority Tamils from voting against President Mahinda Rajapakse in next week's election.

A top opposition official said the sending of reinforcements to Tamil-majority areas was part of a strategy to intimidate voters against backing Rajapakse's main challenger Maithripala Sirisena in the January 8 contest.

While Rajapakse is popular among the majority Sinhalese community, he is despised by many Tamils after overseeing the crushing end to a 37-year separatist rebellion in 2009 which sparked accusations of rights abuses.

Ranil Wickremesinghe, one of Sirisena's main backers, told the Colombo diplomatic corps that he had information about the "use of military officers and military personnel to interfere in the electoral process to prevent people from voting in the (mainly Tamil) north and the east".

Wickremesinghe, who is a former prime minister, said some 2,000 troops in civilian clothing had already been deployed in the north to deter potential voters from going to the polls next Thursday.

The Tamils' largest political party has endorsed Sirisena, accusing the incumbent of failing to pursue reconciliation in the wake of the 37-year conflict against the Tamil Tiger rebels which left some 100,000 dead.

Wickremesinghe narrowly lost the 2005 presidential election to Rajapakse after the Tigers enforced a boycott in the northern war zone.

Wickremesinghe, who is tipped to be a prime minister under a Sirisena presidency, said the opposition would publish the names of military officers assigned to disrupt the vote and warned them to back off.

"I would like to ask those gentlemen (of the military) not to do anything foolish," Wickremesinghe said at the meeting with the diplomats which the media were invited to observe.

There was no immediate comment from the security forces, but the military earlier this week denied that it was campaigning for Rajapakse.

Military spokesman Brigadier Ruwan Wanigasooriya rejected opposition claims that soldiers were distributing material in support of the President.

However, the elections chief last week ordered the army to stop using state money to post Rajapakse's propaganda material to some 200,000 soldiers and their families.

Rajapakse, who came to power in 2005 and is South Asia's longest-ruling leader, called the snap election two years ahead of schedule after his party's popularity dropped 21 points at a local election in September.

His bid to extend his rule has been undermined by a series of defections, with a higher education minister becoming the 25th parliamentary ally to jump ship on Thursday.

"A change is needed in the country," Nandimithra Ekanayake told reporters in Colombo as 17 local councillors also crossed over from Rajapakse's Sri Lanka Freedom Party (SLFP) to join the opposition ranks.

Earlier in the week, the main Muslim political party also defected leaving only a handful of minority community leaders with the SLFP.