NEW DELHI - Indian police on Tuesday killed 12 demonstrators after opening fire on thousands of people demanding the closure of a copper factory due to pollution concerns, a police officer said.

Protesters rampaged for hours in the southern state of Tamil Nadu calling for the closure of the plant owned by British-based mining giant Vedanta Resources.

“We have confirmation of 12 people being killed in police firing. We fear the toll may rise,” the officer told AFP from the state capital Chennai.

Some 20 police were injured in the clashes in Tuticorin, about 600 kilometres (375 miles) south of Chennai. The shootings caused immediate outrage. Rahul Gandhi, leader of the opposition Congress party, said it was “state sponsored terrorism”.

Protesters stormed the office of the top local administrator and set it ablaze after they were denied permission to hold a rally at the smelting plant.

The police officer said efforts to disperse the 5,000 strong rock-throwing group through a baton-charge and tear-gas volleys failed. Police then fired live ammunition, he added. Another officer said more than 110 vehicles were burnt during the disturbances that lasted nearly four hours. Tamil Nadu chief minister Edappadi K. Palaniswami ordered an inquiry into the shootings but defended the action of police.

“The police had to take action under unavoidable circumstances to protect public life and property as the protesters resorted to repeated violence... police had to control the violence,” he said in a statement.

The families of each victim would be offered one million rupees ($14,700) compensation, he added. Residents have been protesting for months against the plant run by a Vedanta subsidiary, Sterlite Copper.

The protests have intensified after Vedanta, owned by an Indian billionaire but with its head office in London, sought to expand the plant. The plant — which is currently non-operational — has a 400,000-tonne annual capacity.

It was shut briefly after an alleged gas leak in March 2013 that left hundreds with breathing difficulties, nausea and throat infections. India’s federal green court allowed it to be reopen.

“Entering the office and violence is not acceptable... firing [opening fire] became unavoidable,” fisheries minister D Jayakumar told reporters in Chennai, formerly known as Madras. He said nearly 3,000 troops had been brought in from neighbouring districts to control the situation.

Protesters say the plant is polluting ground water in the area and threatens the fishing industry.

The plant has been shut for more than 50 days due to alleged non-compliance with environmental rules.

Vedanta has denied all the allegations against it and says it plans to double the capacity of the plant. The company has not commented on Tuesday’s violence.