A regional human rights group sharply criticised Sri Lanka on Thursday over plans to court martial the island's former army chief on charges of conspiring against the government.
"The case is being pursued in military courts, thus avoiding the country's ordinary courts and depriving him of due process," the Hong Kong-based Asian Human Rights Commission said in a statement.
"This is also depriving the public of information on his case," it said.
Retired general Sarath Fonseka, who challenged President Mahinda Rajapakse at the ballot box in January, was arrested on Monday.
The government has yet to spell out the charges against Fonseka, but defence ministry spokesman Keheliya Rambukwella told reporters the general had colluded with opposition politicians last year while still heading the army.
The case triggered clashes between thousands of Fonseka supporters and ruling party activists outside the capital's Superior Courts complex on Wednesday, which were broken up by riot police using tear gas and water cannon.
The AHRC said Sri Lanka was engaging in an "alarming attempts to repress the avenues of the political participation of the opposition and the entire population in general."
Fonseka has challenged his arrest and detention by the military in the supreme court. The case is to be taken up on Friday.
Opposition parties who backed Fonseka have demanded his immediate release and said they feared for his safety in military custody.
The increase in tensions came as President Rajapakse dissolved parliament on Tuesday and called snap legislative elections for April 8.
Rajapakse was returned to power for a second term last month after easily defeating Fonseka in a bitter presidential election battle.