VARANASI (AFP) - India's prime minister-elect Narendra Modi on Saturday vowed to clean up the holy river Ganges, sacred to millions of Hindus but seeped in filth due to years of apathy and neglect.

The Hindu nationalist leader of the Bharatiya Janata Party - who is set to be sworn in as premier next week - made a fervent plea to the people of Varanasi, the city through which the river flows, to help restore the pristine glory of the Ganges.

"When I see the pitiable condition of the Ganges I feel pained," Modi told his supporters on the banks of the river where he offered prayers as thanksgiving for his party's landslide win in the parliamentary elections.

"The need of the hour is to restore the glory of the Ganges. Today Mother Ganges is calling us... to make the river clean once again," he said to a roaring applause from hundreds of supporters.

The belief that the Ganges washes away sins entices millions of Hindus into the river each year.

For devotees, the river is always clean and pure - even if its waters are a toxic stew of human sewage, discarded garbage and factory waste.

Modi, 63, said it was destiny that he contested and won his parliamentary seat from Varanasi because it gave him an opportunity to revive the river.

"The Ganges keeps saying that some son of mine should come and take me out of this filth. It was my destiny to serve mother Ganges," he said.

Ganges water is considered by many to be blessed, and has for centuries served as an essential component of Hindu ceremonies, from childbirth to death - when ashes are often scattered in the river after cremations.

Modi also urged his supporters to work towards making the city clean.

Garbage dumps, open sewage and drains are common sights in Varanasi despite its draw as one of the major pilgrimage sites for Hindus.

"You must think that I have gone mad, that I am not good enough to be the PM because I am only talking about garbage," Modi said.

"But I tell you cleanliness is what is going to salvage this great city and the great river," he said.