Over half a million people took part in a colourful inaugural ceremony in the town of Nashik on Wednesday.
Security was tight after Monday's bomb blast in nearby Bombay, with more than 3,000 policemen deployed at the 12-day festival.
The Kumbh Mela (Grand Pitcher Festival) is held four times every twelve years alternately in four holy sites - Nashik, Ujjain, Hardwar and the main one at Allahabad.
Saffron clad priests and holy men mingled with ordinary devotees in the streets of Nashik on Wednesday, singing and dancing in procession towards the river.
There they plunged into the swollen waters of the river, after days of heavy rain.
For many pilgrims it was a rare chance to take part in the Kumbh.
"After 12 years I have got the chance. I do not know whether I will be alive for the next fair which will come after 12 years," Shakuntala Kasari told the AFP news agency.
"Hindus wait for this day and I am lucky to have dipped myself in this holy water."
Southern 'Kumbh'
But millions of Hindus are also gathering downstream for a parallel festival.
The Godavari Pushkarma also known as the "Kumbh of the South" is being held at Rajahmundry in the southern state of Andhra Pradesh.
The BBC's Omer Farooq, who is in Rajahmundry, says the state government has undertaken massive preparations to ensure the event passes off smoothly.
The authorities have built 170 new facilities for pilgrims to bathe, perform religious rituals and change their clothes.
They have also renovated 350 temples.
Religious significance
The festivals stem from a Hindu myth that gods spilt four drops of the nectar of immortality at four points in India during a fight with demons.
"As Jupiter moves from Cancer to Leo, it brings Pushkaram [the sacred 12 days] to the river Godavari," Andhra Pradesh's Endowment Commissioner D Suryanarayana Murthy told the BBC.
"It returns only after 12 years."
The biggest Mela happens in Allahabad on the river Ganges in northern India.
Over 100 million people are thought to have attended the last festival in Allahabad in 2001.
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