The devotees are marking an auspicious bathing day on Monday at the Kumbh Mela religious festival in Haridwar city in the Himalayan state of Uttarakhand.
The Kumbh Mela takes place every 12 years and the venue is chosen from amongst four cities, including Allahabad, Haridwar, Nasik and Ujjain.
On Monday, India logged more than 168,000 new cases, overtaking Brazil to become the country with the second-highest number of cases globally.
With 13.4 million cases, Brazil is now at number three.
Health experts had appealed for the Kumbh Mela festival to be cancelled, but the government went ahead saying safety rules would be followed.
The government had earlier said that only people with Covid negative reports would be allowed at the festival and strict measures like social distancing would be followed.
But a number of people, including top saints, have already tested positive.
This has given rise to fears that Monday's bathing day will help the infection spread faster among the devotees and that some of them could also take the virus back to their cities and villages in other parts of the country.
Measures like night curfews and limited lockdowns have been taken but the green light given to the Kumbh Mela festival has baffled experts.
India has given more than 100 million doses so far but the pace needs to further pick up to halt the spread of the virus, experts say.
How to plan a festival for 100m people.
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