Thousands of sadhus in their trademark saffron-coloured clothes held protests for a second day Monday, demanding that the river be cleaned up before the next auspicious bathing day on Sunday, a Hindu leader said.

"The water in (the) river is so dirty that no one can take a dip. It is dark red whereas the Ganges used to be bluish green," said Shankaracharya Vasudvanand Saraswati, who heads the main Hindu monastery in the holy city of Allahabad, where the festival is taking place.

"If the government takes no corrective measures we will have no option but to boycott the (Ardh Kumbh) festival," he told AFP by telephone.

Billed as one of the world's biggest human gatherings, the festival started last week with Hindus taking a dip at the confluence of two sacred rivers -- the Ganges and the Yamuna.

The Ardh Kumbh mela, held every six years at Allahabad to mark a mythical battle between gods and demons over a pitcher or kumbh of the nectar of immortality, was expected to draw as many as 70 million people over the next six weeks.

Devotees believe the holy waters wash away sins, liberate them from a continuous cycle of birth and reincarnation and guarantee immortality.

"The pilgrims come here to wash away their sins but after a dip here, they may carry skin diseases with them," said Hari Chaitanya Brahmachari, another powerful Hindu figure who runs the monastery in Varanasi a city on the Ganges.

Brahmachari has filed a court case against the state government of Uttar for not keeping the Ganges clean.

State officials said they will release fresh water via canals and dams to help improve water quality for the mela.

The Ganges, which rises in the Himalayas, is polluted by industrial effluent and human waste as it winds through the Indian plains before emptying into the Bay of Bengal.

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