Protesters march near the copper plant owned by London-based Vedanta in India's coastal district of Tuticorin on May 22. (Photo supplied)

Four Catholics were among 11 people who died when police opened fire on thousands protesting a multimillion-dollar copper-processing plant which they claim pollutes their environment and drinking water.

At least 20 people including a Catholic priest are in hospital with bullet injuries after Tamil Nadu state police fired at about 20,000 people gathered at the plant owned by London-based Vedanta in the coastal district of Tuticorin, a Christian center in southern India, on May 22.

"The death toll may go up as several are critically injured," Father Norbert Thomas, chancellor of Tuticorin Diocese, told ucanews.com.

He said police fired indiscriminately to disperse the crowd after it turned violent on the 100th day of a protest.

State Fisheries Minister D. Jayakumar told media that police resorted to firing when the crowd went on a rampage near the office of the district collector, the highest state official.

Police had imposed prohibition orders near the office and protesters had no permission to organize the march. "The use of force by the police was unavoidable" to disperse the crowd, Jayakumar said.

Bishop Yvon Ambroise of Tuticorin told ucanews.com that firing was indiscriminate. There was an effort to claim the protest was instigated by the church but it is was a people's protest because they were seriously affected by it, he said.

Vedanta's subsidiary Sterlite Copper has operated a smelter in the area for 25 years with an annual capacity of 400,000 tonnes and is seeking renewal of its license, which expires this year.

People have been protesting for the past three months and on the 100th day they organized a march to the office of the district collector, said a statement from the bishop.

The protest followed the non-violent principle of Mahatma Gandhi, the father of the Indian nation, and the five-kilometer march was also also peaceful, the statement said.

Police baton-charged people near the office and some responded by throwing rocks at police, which led to police firing shots and the deaths of innocent people, the statement said.

"The whole thing started after police action," said Father Thomas, adding that firing was "so brutal" that several women and children were also injured.

One bullet passed through the mouth of a 17-year-old girl student, killing her on the spot.

Among the injured is Father Leo Jayaseelan. A bullet went through his stomach but he was operated on and is stable now, the chancellor priest said.

He said the polluting plant operates within the city limits and people have been adversely affected by toxic fumes. In recent years, the area has reported increased cases of cancer.

The plant comes under the area of a parish and affects 19 parishes in the city, which together caters to about 100,000 Catholics.

"It is natural for the very traditional Catholics here to come to the church and consult in the parish when they face social issues," Father Thomas said. "However, the protest has nothing to do with religion. It is a people's protest."