"He will sacrifice his life for the river, so I hope the government opens its eyes and accepts his demands," said Rajendra Singh. "Nothing has been done to revive the river which is dying a slow death."
Singh said Agrawal was on a "spiritual mission" to demand the construction of hydroelectric dams on the Ganges is banned and the water is cleaned of the industrial and human waste discharged into the river every day.
Agrawal was rushed by air ambulance from the holy city of Varanasi on the banks of the Ganges to hospital in central Delhi on Monday.
Large sections of the 2,500-kilometre (1,500-mile) river, which Hindus believe washes away all sins, are polluted by sewage, discarded garbage and factory waste as it crosses the northern Indian plains.
Last year, another holy man, Swami Nigamanand, had died in the pilgrimage city of Haridwar after a fast to stop illegal mining of the Ganges riverbed.
Agrawal's protest has gained strong public support as well as endorsements from popular activist Anna Hazare and politicians such as Narendra Modi, a senior member of the opposition Bhartiya Janata Party.