The deaths came on the 100th day of demonstrations against the plant, which environmentalists and residents say is contaminating water sources.

A protester was killed by a rubber bullet in southern India on Wednesday, just a day after 10 were shot dead when police opened fire on a rally demanding the closure of a copper plant over health and environmental concerns, officials said.

The shooting, which also injured 80 people, sparked intense criticism. Amnesty International said police had "many questions to answer" and those responsible should be brought to justice.

Police fired rubber bullets and sent volleys of live ammunition overhead, officials and witnesses said, after protesters in the port city of Tuticorin hurled home-made bombs and pelted officers with stones.

Another police officer said a 22-year-old man had died. Police earlier said 12 people had died, but later revised the toll. P. Mahendran, superintendent of Tuticorin district police, said 18 officers were wounded in the clashes.

The demonstrators are demanding the closure of a copper plant on the outskirts of Tuticorin in the southern state of Tamil Nadu owned by British mining giant Vedanta Resources, which they say is causing environmental damage.

A video on social media showing a police officer atop a bus pointing an assault rifle at the crowds has fuelled fresh anger. Witnesses said demonstrators set fire to a police bus and ransacked a liquor shop.

TV footage showed police in riot gear patrolling streets littered with stones and burnt tires. Traders shut down shops as authorities imposed a curfew in parts of the city.

Tamil Nadu chief minister Edappadi K. Palaniswami ordered a judicial inquiry into the shootings, but defended the police response.

M.K. Stalin, leader of main Tamil Nadu opposition party the DMK, said police were guilty of "atrocities." "Mass Murder of Innocent People," he tweeted Wednesday. "Who ordered the police firing on protesters? Why were automatic weapons used to disperse the crowd and under what law is this permitted?"

Rahul Gandhi, the national leader of the opposition Congress party, described the deaths as "a brutal example of state-sponsored terrorism." "These citizens were murdered for protesting against injustice," he said.

The Madras Union of Journalists complained of police heavy-handedness. "A group of policemen and rogue elements snatched and damaged the cameras," said journalist S. Raghunathan.

The plant is currently closed while Vedanta's Sterlite Copper subsidiary seeks a new license so it can be expanded.

The protests intensified after Vedanta, owned by Indian billionaire Anil Agarwal but with its head office in London, sought to double the 400,000-tonne annual capacity of the plant. It was shut briefly after an alleged gas leak in March 2013 that left hundreds with breathing difficulties, nausea and throat infections.

However, the company maintains that it adheres to environmental standards and said it was the victim of "false propaganda" about its operations. Agarwal said on Twitter earlier this month that his company was the victim of a foreign conspiracy aimed at keeping India reliant on imports. He did not name any specific countries or companies.

Tamil Nadu is one of India's most industrialized states and similar protests over environmental concerns have turned deadly in the past, including in Tuticorin.

In Thootukudi, Vedanta's copper plant has been shut for more than 50 days and will remain closed until at least June 6 because the local pollution regulator has said the facility is not complying with environmental rules.

Several cases have been filed against the plant since it started in 1996, and India's top court in 2013 fined it about US$18 million for breaking environmental laws. The next year, Vedanta lost a battle to mine bauxite in the a lushly forested area, Niyamgiri hills in Odisha state, that the Dongria Kondh tribe there considers sacred.