from voting after the main Tamil political party backed opposition candidate Maithripala Sirisena.
The Tamils make up the largest minority on the island, and may be the deciders in Thursday’s election if, as appears likely, the majority Sinhalese vote is split between Rajapakse and Sirisena.
South Asia’s longest-serving leader had appeared politically invincible after his forces crushed the Tamil Tigers in 2009, ending a decades-long conflict.
But his second term has been dogged by accusations of corruption, including undermining the independence of the judiciary and lining the pockets of political cronies through lucrative contracts.
Sirisena’s surprise decision to stand against him has turned what might have been a walkover into a bitterly-fought contest.
Northern Sri Lanka remains heavily militarised after the war.
An international poll monitor who asked not to be named told AFP he had to intervene to secure the withdrawal of troops from a roadblock in Wanni on Wednesday.
Deshapriya said he had received assurances that troops had only been placed on stand-by, although he said they could be deployed in the event of post-election unrest to help protect vital installations.
Deshapriya said the security forces had no role in the elections and police were capable of protecting 49 counting centres and 12,314 polling booths, at which 15 million people are eligible to vote.
He said police had been ordered to “shoot in the head” if anyone tried to disrupt the election.
Police have said that 65,000 officers armed with an automatic assault rifles are being deployed around the country.
The election commissioner also said he had issued warnings to both state and private media organisations for violating the rules on broadcasting campaign materials.
State television networks on Wednesday tried to overturn a court injunction banning them from broadcasting an interview with the president because it breached election laws.