The human rights group, Amnesty International, says it is gravely concerned about Sri Lanka's decision to reinstate the death penalty.
Amnesty said it had urged the government to reconsider.
President Kamaratunga reinstated the death penalty on Saturday, for rape, murder and drug dealing.
Capital punishment has not been carried out in Sri Lanka for almost 30 years, with death sentences in effect being commuted to life imprisonment.
In a statement, Amnesty said there was no proof the death penalty was a more effective deterrent to crime than imprisonment.
President Kumaratunga's statement came a day after a High Court judge was shot dead in the capital, Colombo, by a suspected drug gang.
The judge was known for handing down death sentences to major figures in Sri Lanka's criminal world.