David and Goliath are familiar characters in Tuticorin. The south Indian city, home to one of the country’s oldest Christian communities, has been fighting a giant. Days after 13 protesters were fatally gunned down by police, including a teenage girl, activists won a key victory over one of the world’s largest mining companies.

For more than two decades, Sterlite, a subsidiary of the London-listed Vedanta Resources, has been operating a copper smelter on the outskirts of the city. Over the same period, activists have been arguing the site is fouling Tuticorin’s air and water, and has caused unnaturally high cancer rates in surrounding villages.

Last Monday, Fatima Babu, a leader in the movement, had been touring affected villages when she got the news the Tamil Nadu chief minister was ordering the plant permanently closed.

“Until last week I never thought it could happen so easily,” she told the Guardian minutes after finding out, as people around her wept and cheered. “Only after the loss of lives did I think things could take a turn. The martyrs have done it.”

Victory did not come cheap. On 20 May, tens of thousands of residents turned out for the largest protests against Vedanta ever staged in the city. Police allege the protesters became violent, hurling stones, torching cars and threatening to burn public buildings. Officers opened fire, killing 13 people over two days, including a teenage girl.

It was a bloody climax to a 23-year struggle that began in fishing boats on the Indian ocean, frequently faced defeat, and was at times kept alive by Babu alone, a teacher who spent years pursuing the campaign in the evenings after work.

Against them was one of India’s most powerful corporations, run by the billionaire Anil Agarwal, which was frequently accused of running roughshod over the small communities that surround its bauxite mines, alumina refineries and copper smelters. Though it does most of its business in India, Vedanta is listed on the London stock exchange, where its most recent half-yearly profits were $1.7bn.

In Pandarampatty, a village not far from the plant, residents claim 200 people have been diagnosed with cancer since 1996. That was the year the Vedanta plant opened in Tuticorin, after pressure from villagers in Maharashtra state forced a shift from its original site there.

Indian police officials wield bamboo sticks in clashes with protesters in the southern Indian city of Tuticorin on 22 May, during a rally to demand the closure of a Vedanta copper plant. Photograph: Arun Sankar/AFP/Getty Images

The morning after the announcement of the plant’s closure, few in the village were celebrating. Rosalin, who like others would only give her first name, was still recovering from her latest dose of chemotherapy. Last year she was diagnosed with breast cancer, and underwent a mastectomy nine hours’ drive away in Chennai. “It doesn’t feel like we’ve won,” she said. “I’ll be living with this for the rest of my life.”

Jasmine, a friend, agreed: “We feel this [the closure announcement] was a publicity stunt from the government. It was just for TV. In six months, they will let them re-open.” Vedanta says it has not decided whether to appeal against the government’s decision to close the plant.

A few metres away, their neighbour, Mary, lay on a charpoy bed in the shade, taking shallow breaths. She had been diagnosed with emphysema, which she blamed on decades of breathing air polluted by the chimneys of the nearby factory.

Whether Vedanta is to blame for their illnesses is difficult to establish. The plant has been temporarily shut several times after suspected leaks of noxious chemicals. In 2013 it was closed again and Vedanta was fined £10m after a court ruled the plant was operating without the consent of environmental authorities. It re-opened after a team of experts judged it was not polluting beyond its legal limits.

A Tirunelveli medical college study 10 years ago found villagers around the plant had higher rates of asthma, bronchitis, and ear, nose and throat infections, and that iron levels in their groundwater were up to 20 times higher than acceptable limits.

Vedanta denies any link between its plant and health problems in surrounding villages, maintaining the facility “has been operating within all applicable environmental regulations and standards”.

It says cancer rates in Tuticorin are lower than the state average. “Any allegation linking the incidents of cancer and marine pollution with Sterlite Copper are unfounded,” it said in a statement. “We are a zero liquid discharge company and do not affect the marine ecosystem in any manner.”

Nor do monthly water samples, taken from around the plant by environmental authorities, show any trace of copper, zinc or other pollutants, it says.

Tuticorin is a hub for a number of other polluting industries including fertiliser, thermal power and caustic soda.

The difficulty, say environmental scientists, is that a study has never been carried out looking into the health impact of these industries. The Tamil Nadu government also lacks the expertise to seriously monitor and mitigate most environmental damage.

“There is no capability in this state to undertake a rigorous and scientific environmental impact assessment,” says Jayshree Vencatasan, a scientist and managing trustee at the Care Earth Trust, a conservation group. “We just don’t have organisations with the scientific and technical capability to do it.”

The first in Tuticorin to turn against the smelter were the fishermen, who feared its wastewater was being dumped in the ocean. In 1997, hundreds of fishing boats formed a shield around the port, preventing two Australian vessels from berthing with copper ore.

The same year, activists say gas from the plant leaked into a neighbouring flour mill, hospitalising about 20 people and prompting the government to briefly shut the smelter.

By that time, Babu, a literature teacher at a local college, was spearheading a campaign against the facility, organising protests and trying to fuse disparate concerns about the smelter into a larger movement. “Whenever we heard of some misdeed by Sterlite we would come out in the streets, make a lot of noise, go on fasts,” she recalls.

She maintained her opposition even as the protests died down, running what she calls a “single person struggle” from her home, “hoping and trusting something good would come out of it”.

“The lowest point was in 2013, when there was a gas leak and the entire town was impacted,” she said. “I believed we were finally at the end of the road, Sterlite was going to go, but somehow, it fizzled out. People thought you can’t do anything about this company, it was too powerful for us.

“They got over the impact and went on as if nothing happened.”

Then, in September 2016, Vedanta applied to double the capacity of the plant, making it the largest copper smelter in the country. It ignited latent anger in the city, which Babu says was compounded by mounting health problems in many families. Social media, another new factor, helped spread information and coordinate protests.

Early this year, activists launched a 100-day campaign to stop the government granting Vedanta approval to expand the plant. Its final day was 21 May, when tens of thousands of people marched in the streets and were fired on by police. Outrage over their killings turned the smelter in Tuticorin into a national issue. One week later, its gates were sealed.

In the city’s government hospital this week, dozens of people were still in beds recovering from injuries they sustained during last week’s violence. “All because this one company has polluted our air, water and soil, we’ve had to suffer like this,” one woman, Nandini, says. Her husband lies nearby, his face still bruised from being struck by police clubs. “I will fight more if I have to,” she says.

Additional reporting by Divya Karthikeyan