India's top green court Wednesday gave the go-ahead to a mass festival organised by a world-famous guru to be held on the floodplains of Delhi's Yamuna river, despite fierce opposition from environmentalists.

Green activists had filed a petition saying the three-day World Culture Festival, which starts March 11 and is organised by Sri Sri Ravi Shankar, risks damaging the river's delicate ecosystem.

The National Green Tribunal said it would allow the festival to go ahead given that planning was in the advanced stages, but fined Shankar's Art of Living foundation 50 million rupees ($740,000) for environmental damage.

"It is consistent view of the experts... that floodplains have been drastically tampered with," Justice Swatanter Kumar said Wednesday.

Kumar said the organisers would be responsible for all damage to the environment, biodiversity or aquatic life and that the fine could rise depending on the severity of the impact.

"We impose an environmental compensation, initially of five crore (50 million) rupees," the court stated in a strongly worded order.

Some 500,000 people are expected to attend the festival, the organisers told the court, although earlier advertisements suggested it would draw more than three million visitors.

Prime Minister Narendra Modi is also expected to attend, organisers said, although media reports have suggested he may stay away following the controversy.

The festival is billed as a platform "for spiritual and religious leaders, politicians, peacemakers and artists to spread the message of global peace and harmony in diversity".

The court also criticised several local and national government agencies for allowing the event to be held in the fragile ecological zone.

"All these authorities have failed to exercise due diligence in fulfilment of their public duties," the court said.

It said that the Delhi Development Authority would not be allowed to issue permission for events to be held on the floodplains in future without the tribunal's agreement.

One of the initial petitioners in the case, activist Manoj Misra, described the order as a victory for the environmentalists' campaign.

"What more do you want? We have won," Misra told AFP outside the court.

"All of them including the government organisations have been found guilty and fined by the court. In future, they can't even give permissions for a similar event on the floodplains," he added.

The Yamuna river is the largest tributary of the Ganges, considered holy by Hindus.

Despite high levels of pollution, its floodplains are rich in flora and fauna, with more than 320 bird species and 200 types of plants.