Hundreds of thousands of Indian pilgrims began packing an island in the Ganges on Thursday for a vast religious festival as COVID-19 cases surge nationwide, prompting fears of a superspreader event.

Officials said they expected as many as three million people, including ash-smeared, dreadlocked ascetics, to take a ritual dip in the holy river on Friday, the climax of the annual Gangasagar Mela.

Last April's Kumbh Mela, a similarly colossal religious festival that the Hindu nationalist government refused to ban, was partly blamed for a devastating spike in cases that killed 200,000, according to official tallies, though some experts fear the actual toll was several times that.

With coronavirus case numbers once again ballooning, a Kolkata court last week ruled that the Gangasagar Mela, which takes place on Sagar Island at the mouth of the Ganges in West Bengal, could go ahead.

The state government on Thursday appealed to people to get tested for COVID-19, with West Bengal Chief Minister Mamata Banerjee urging devotees to wear two masks and not "spit on the island, as it spreads the virus".

As with 2021's Kumbh Mela, this week's festival is attracting people from across northern India who, after cramming onto trains, buses and boats to reach the island, will then go home -- potentially taking the virus's highly transmissible Omicron variant with them.

"It's a big challenge amid the raging surge of the COVID-19 variant to maintain social distancing," local district magistrate P. Ulganathan told AFP.

"There is no doubt that such a gathering where people are in close proximity will increase the spread. We are continuously appealing to people to follow COVID-appropriate behaviour."

Amitava Nandy, a virologist from the School of Tropical Medicines in Kolkata, said the government "has neither the facilities nor the manpower" to test everyone attending or impose social-distancing norms.

"A stampede-like situation could happen if the police try to enforce social distancing on the river bank," Nandy told AFP, saying the festival "may end up being the superspreader of the virus in India".

Devotee Sarbananda Mishra, 56, who came from the neighbouring state of Bihar, told AFP that "faith in God will overcome the fear of COVID-19. The bathing will cleanse them of all their sins and bring salvation."

"Death is the ultimate truth. What is the point of living with fear?" said Mishra, a schoolteacher resting at one of the many camps in nearby Kolkata waiting to go to the festival.