The Kumbh Mela, the Hindu religious festival described as the largest gathering of mankind in history, is contributing to the alarming levels of pollution killing the Ganges, the faith's holiest river, officials have warned.

Campaigners called on the government and worshippers to take action to save them from chemical pollution and human sewage as millions of devotees immerse themselves on Monday at the confluence of the Ganges and the Yamuna.

Despite government measures to reduce the human and industrial waste from leather factories upstream, the impact of more than 80?million worshippers bathing in the river and camping on its banks had raised organic pollution to dangerous levels.

Tests carried out by Uttar Pradesh's state pollution control board found levels of biochemical oxygen demand (BOD), which measures organic pollution, were at more than seven milligrams per litre - double the maximum acceptable level - after the first day of the mela. On that day last month an estimated 10?million bathed in the Ganges.

By the time the festival ends next month, up to 100?million people will have bathed in the river.

Uttar Pradesh has built a vast temporary city of campsites, police stations, hospitals and shopping centres to cope with the numbers, including its own sewage systems. It pledged to build 35,000 lavatories and raise the water levels in the Ganges and Yamuna to wash away waste quickly.

But as 10?million people gathered there yesterday in preparation for today's 4am immersion, there were signs that efforts to keep the tent city clean had reached their limits. Hindu ascetics in saffron robes and matted dreadlocks defecated openly on the roadside as other worshippers made their way to the river banks.

Dr Suresh Dwivedi, head of health and sanitation for the festival, denied that the government had not made enough sanitary provision but admitted that there was a problem of "open-air defecation".

"It is a social problem in our country," he said. "A lot of people come here and defecate in the open, so we constructed pits."

In the rivers, tons of rotting marigolds, coconuts and other offerings formed a scum on the surface, as worshippers drank the holy water. The river is polluted by the ash from thousands of cremations, rotting animals, and the human waste of millions of people. Of the 660?million gallons of sewage produced by cities along the river, only a third is treated.

Regardless of the pollution, Hindus believe that bathing in the Ganges will purify them and help them to achieve their -desired Moksha, an end to continual reincarnation.

Swami Chidanand Saraswati, the head of the Parmarth Niketan ashram, the largest in the holy city of Rishikesh, has dedicated his order to saving the river. Hindus, he said, focused too much on their creator but did not care enough for his creation.

"What's the point in coming here and having one dip in the holy river but not taking care of it? We need waste management, sewage management, the sewage should be in a canal, but it is not being done," he said.