NEW DELHI (REUTERS) - Prime Minister Narendra Modi attended the opening on Friday of a giant music and dance festival that has been criticised for environmental damage and the possible safety risks of its enormous stage.

Large crowds braved heavy rain for the start of the event, crossing army-made pontoon bridges across Delhi's main river, the Yamuna, to the "World Culture Festival", where 35,000 dancers and musicians will perform to an expected audience of millions over three days.

Modi, the star guest at the opening ceremony, sat next to organiser Sri Sri Ravi Shankar, a well known guru, on a specially built VIP stand, as hundreds of holy men chanted in harmony and women dressed in red and gold danced to drum beats on the stage below.

The festival has pitted Ravi Shankar against environmentalists, concerned about damage to the nature-rich floodplain, and the Delhi police, who said the 3ha stage - which organisers say is the world's largest - lacked a safety certificate.

India's top environmental court this week fined Ravi Shankar's organisation 50 million rupees (S$1 million) and questioned why authorities had allowed the festival to be held in such an environmentally sensitive area, but it did not stop the event going ahead.

Green groups say the event will destroy the biodiversity of the 400ha site by ripping up vegetation, flattening land and altering the river's flow.

Ravi Shankar has rejected criticism and promised to leave the area better than he found it.

Delhi police expressed concern about whether organisers were equipped to manage such a large number of people, warning of the potential for "utter chaos".

The Yamuna, a tributary of the Ganges and the source for much of Delhi's water, is revered by many Hindus who bathe in it, despite high levels of industrial and human waste.