A supporter of former army commander General Sarath Fonseka (R) bleeds from his face after being hit by a rock during a protest rally in Colombo yesterday, held by opposition parties to protest the arrest of former army chief Sarath Fonseka. Photo: AFP

Sri Lanka's opposition parties demanded yesterday the immediate release of former army chief and defeated presidential candidate Sarath Fonseka, warning that he faced "assassination" in custody.

In a joint statement, the parties, which had backed Fonseka's failed challenge to President Mahinda Rajapakse in January elections, said they feared for the retired four-star general's life following his arrest.

"We have good reason to believe that the extra-judicial arrest of General Fonseka may be followed up with assassination while in custody," the statement said.

Fonseka was taken into custody by military police on Monday night. The defence ministry says he will be court-martialled on charges of conspiring against the government while serving as head of the army.

Fonseka's family had been told Tuesday evening that he was being held at the naval headquarters in Colombo. His wife and a lawyer were allowed to visit him.

Fonseka quit the military in November after falling out with Rajapakse and went on to challenge his former commander-in-chief at the ballot box.

Shortly after the January 26 vote, which Rajapakse won comfortably, the government accused Fonseka of plotting a coup to overthrow the president and assassinate members of his family.

And earlier this month, Rajapakse sacked a dozen senior military officers described by the defence ministry as a threat to national security. More were arrested from Fonseka's office.