MUMBAI (Reuters) - One of India’s most prominent environmental activists has died at the age of 86 after more than 15 weeks of a hunger strike to protest against government inaction on cleaning up the Ganges.

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The death of G.D. Agarwal, who held a PhD in environmental engineering from the University of California in Berkeley, prompted an outpouring of grief and tributes from activists.

“His demise has shut one of the leading voices of criticism of the government on the Ganga pollution,” said environmentalist Rakesh Jaiswal. “He was one of the most important figures in this fight.”

The Ganges, worshipped by Hindus, is India’s largest river system and one of its most polluted. Prime Minister Narendra Modi came to power in 2014 with a pledge to clean up the 1,570-mile-long river, used for water by 400 million people, but increasingly choked with domestic and industrial waste.

A flagship five-year project he launched in 2015 has fallen flat, critics say. Results of a federal audit released in Dec. 2017 revealed lapses in planning and financial management of the scheme and said under a quarter of the funds for the program had been spent in two years.

Agarwal began his fast on June 22 in the northern Haridwar city, demanding a law to protect the river and the scrapping of construction of hydroelectric projects along its banks that have destroyed its natural flow.

In a letter to Modi in August, he threatened to fast unto death unless action was taken.

Modi said on Twitter he was “saddened” by Agarwal’s death. “His passion toward learning, education, saving the environment, particularly Ganga cleaning, will always be remembered. My condolences.”

India’s opposition Congress party leader Rahul Gandhi also paid tribute. “To save the Ganga is to save the country. We will take his fight forward,” he wrote on Twitter.

Ravi Kant, director of the AIIMS hospital in the northern city of Hrishikesh, on the banks of the Ganges, said Agarwal died of a cardiac arrest on Thursday.

He had been forcibly taken to the hospital by police a day before, hours after he stopped drinking water. Television footage showed Agarwal, clad in a saffron robe, being picked up by police officers along with the chair he was sitting on, as he kicked his legs in protest.

Some activists on Friday demanded an independent investigation into Agarwal’s death.

“I don’t think the government made any honest effort to fulfill his demands,” said Ravi Chopra, an environmentalist.