The United National Party (UNP) criticised the army's plans for an internal military probe into the death of three civilians and wounding of 50 others during a protest at Weliweriya village near Colombo last week.
"We cannot accept the government getting one of its henchmen to probe the killings," UNP general secretary Tissa Attanayake told reporters in Colombo. "Innocent civilians have been massacred.
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The military opened fire Thursday on thousands of unarmed residents who were protesting against the contamination of their ground water supply, allegedly by chemical waste from a rubber glove factory.
Attanayake said the attack was a direct assault on democracy and people's right to peaceful protest.
He said the opposition was pressing for an "independent international inquiry" into the incident, a demand echoed by London-based Amnesty International.
The rights group said Sri Lanka's army should not be allowed to investigate itself over the attacks against civilians