Mr Tissainayagam was arrested in March
Human Rights Watch has called on Sri Lanka's government to free prominent Tamil journalist, JS Tissainayagam.
Mr Tissainayagam was arrested in March and charged with inciting violence in articles in his magazine, the North Eastern Monthly, which has been shut.
He has also been accused of aiding terrorist organisations through raising money for the magazine.
The international rights group said Sri Lanka was shamefully using anti-terror laws to silence peaceful critics.
Human Rights Watch called on the authorities to release Mr Tissainayagam from prison in Colombo, where it says he has been threatened by other inmates.
It also asked the government to drop all charges against him.
Mr Tissainayagam has for some years been an outspoken critic of the government, especially in relation to the country's civil war.
He says the security forces have deprived civilians of food, medicine and fuel.
Human Rights Watch has also called for the release of the Tamil publisher, N Jasiharan, and his wife - also arrested in March.
It says that none of the three detainees was given proper access to their lawyers and that, in some instances, police officers were present during legal discussions.
There has been no word so far from the Sri Lankan authorities.