NEW DELHI (Reuters) - Indian environmentalists are aghast that a huge cultural festival is to be held on the floodplain of Delhi’s main river from Friday, warning that the event, and the 3.5 million visitors expected, will devastate the area’s biodiversity.

The “World Culture Festival”, organized by one of India’s best-known spiritual gurus, Sri Sri Ravi Shankar, spreads across 1,000 acres (400 hectares) on the banks of the Yamuna. It features a 7-acre stage for 35,000 musicians and dancers, newly built dirt tracks and 650 portable toilets.

Green groups accuse organizers of ripping up vegetation and ruining the river’s fragile ecosystem by damaging its bed and disrupting water flows. They want authorities to cancel the event and avert further harm.

“This land is not meant for any of those things. The biodiversity of the land has been completely destroyed,” said Anand Arya, one of several environmentalists who petitioned India’s top green court.

“Where will the sewage and the excrement go? All across the floodplains!” he said, adding that the waste left by visitors would endanger a nearby bird sanctuary.

The National Green Tribunal on Wednesday ruled that the event could go ahead but fined Ravi Shankar’s Art of Living Foundation 50 million rupees ($744,000).

Prime Minister Narendra Modi, a yoga devotee like Ravi Shankar, was due to attend Friday’s opening, but it is not clear whether he will do so after the event sparked such uproar - and not just among environmentalists.

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Delhi police have warned of “utter chaos” at the event unless safety lapses are tackled, the Indian Express said, citing a March 1 letter to the federal government saying the stage lacked a structural stability certificate.

Farmers who plough the banks of the river amid Delhi’s urban sprawl also accused organizers of forcing them off the land.

Ravi Shankar, who enjoys a cult following in India and abroad, has rejected the criticism, saying he should be rewarded for hosting the event beside one of India’s most polluted rivers.

Saraswati Akshama Nath, the lawyer for his organization, said approvals, including safety certificates, had been given in December before construction began, and the structures would be removed after the three-day festival ends.

“Consent was given to us by all the authorities,” she told Reuters. “We have only used eco-friendly material.”

At the site, builders were scrambling to complete what they say is the world’s biggest ever performing stage. It can accommodate a symphony orchestra of 8,500 and 20,000 performers, said Prasana Prabhu, a trustee of Ravi Shankar’s foundation.