Sri Lanka’s main opposition party, the United National Party (UNP), on Tuesday called for an international probe into last week’s military crackdown on a protest against poor water quality which killed three and injured nearly 50.

“We call for an international investigation. We can’t trust a probe headed by a government henchman,” said UNP general secretary Tissa Attanayake.

The call for the probe came as the London-based rights group Amnesty International said that the Sri Lankan government should not be allowed to conduct its own probe.

“The Sri Lankan army should have never been policing unarmed demonstrators in the first place,” Amnesty said in a statement.

The Lankan Army has already constituted a five-member board last week to investigate all allegations against the military, including the alleged assault on civilians and the killing of three protesters including a 19-year-old and a 17-year-old boy in Weliweriya — a small town in the country’s northwest.

Mr. Attanayake said it was a question about the level of democracy in the island as people’s right to democratic protest had been challenged. “This will add a black patch to the government in the international eyes,” he added.

Residents of Weliweriya alleged that their ground water has been contaminated by acid waste, dumped by a nearby rubber gloves manufacturing plant. They have been demanding action for several weeks, leading to Thursday’s police and military crackdown.

Television footage showed that soldiers charged into protesters who hurled debris at them during the protest.

Villagers, who had blocked the vital A1 highway, alleged that the Army assaulted them and opened fire on them.

UNP leader Ranil Wickremesinghe asked for an independent probe on the Army’s action in Parliament. The Leader of the House, Nimal Siripala de Silva, assured that government would conduct an inquiry.

The Army had earlier defended its action claiming it was protecting itself against an organised gang which had incited the residents to protest.