"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