Sri Lanka's ex-army chief turned opposition leader appeared at a court martial Tuesday that could see him jailed for up to five years if convicted on charges he says are politically motivated.

A three-member panel of two-star generals began hearing the case against Sarath Fonseka, who is charged with making irregular procurements and engaging in politics while he was still commander of the army.

"The proceedings are under way at the naval headquarters in Colombo," a source with knowledge of the trial told AFP, asking not to be named.

"General Fonseka is present and he has raised a preliminary objection to the three-member panel trying him," the source said.

The military stepped up security in the capital ahead of the hearing and dozens of Fonseka supporters were dispersed with tear-gas in a suburb of Colombo on Tuesday as they tried to stage a demonstration, police said.

Fonseka faces two charges relating to politics and military procurement rules and they will be heard separately by the same panel, which is conducting its hearings behind closed doors.

"There will be about 35 witnesses and the process can take a few weeks, if not months," a separate defence source, who declined to be named, told AFP. "On conviction he could face a jail term ranging from two to five years."

Fonseka, who was arrested by the military on February 8, two weeks after he lost a presidential election to incumbent Mahinda Rajapakse, has described the proceedings as part of a political vendeta against him.

Fonseka's spokesman, Anura Kumara Dissanayake, said the four-star general was convinced that the case was aimed at preventing him from campaigning for the April 8 parliamentary elections in which he is a candidate.

"The general believes that this is a politically motivated move to keep him out of the election campaign," Dissanayake said.

Fonseka and Rajapakse, once allies who last year ended the country's 37-year civil war, are now biter enemies after their political fight

The 59-year-old retired general has refused to cooperate with military investigators, arguing that they have no right to prosecute him under military law after his retirement in November.

Reporters were not allowed access to the military hearing.

Fonseka was arrested after senior government figures declared that he was planning a military coup and had conspired to assassinate the president, but none of these more serious charges has been brought against him.