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 River.

Key points: The prominent environmental activist started fasting on June 22, 2018 to protest pollution in the Ganges

The prominent environmental activist started fasting on June 22, 2018 to protest pollution in the Ganges India's longest river is sacred to Hindus, and provides water to 400 million people

India's longest river is sacred to Hindus, and provides water to 400 million people Mr Agarwal was taken to hospital by police the day before his death, after he had given up water

The death of GD 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," environmentalist Rakesh Jaiswal said.

"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 2,500-kilometre river, used for water by 400 million people, but increasingly choked with domestic and industrial waste.

The Ganges is India's longest river and is sacred to Hindus, but it has become increasingly polluted. ( AP: Rajesh Kumar Singh )

A flagship five-year project he launched in 2015 has fallen flat, critics say.

Results of a federal audit released in December 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.

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Mr 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 Mr Modi in August, he threatened to fast unto death unless action was taken.

Mr Modi said on Twitter he was "saddened" by Mr Agarwal's death.

"His passion towards learning, education, saving the environment, particularly Ganga cleaning, will always be remembered. My condolences," he said.

India's opposition Congress party leader Rahul Gandhi also paid tribute on Twitter.

"To save the Ganga is to save the country. We will take his fight forward," he said.

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

He had been forcibly taken to the hospital by police a day before, hours after he stopped drinking water.

Footage shown on Indian TV showed Mr 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.

Activists held a vigil for Mr Agarwal, and are now demanding an investigation into his death. ( Reuters: Anushree Fadnavis )

Some activists have demanded an independent investigation into Mr Agarwal's death.

"I don't think the Government made any honest effort to fulfil his demands," environmentalist Ravi Chopra said.

Reuters