In what has been billed as the biggest human gathering on earth, more than 30 million people in India are expected to take a "holy" dip in the city of Allahabad on the most important day of the Maha Kumbh Mela festival.

Millions will bathe at the "Sangam", the auspicious confluence of the rivers Ganges and Yamuna. On the eve of "Mauni Amavasya" day, the holiest of the 55 days of Maha Kumbh, 12 million pilgrims have already arrived at the spot.

"All of them are staying back for Sunday's bathe. We seem to be set for a historic turnout. I have been patrolling on horseback for a better view of the crowds. I can only see a sea of humanity everywhere," said the police chief overseeing the festival security, RKS Rathore.

Nearly eight million people took a dip on the first day of the festival. Kumbh Mela is observed every 12 years while this year commemorates Maha Kumbh Mela, which happens only once every 144 years.

The festival began on 14 January and ends in March. Since the opening day, nearly 12 million pilgrims have bathed in the rivers.

Security measures have been tightened since the festival's most important day comes in the wake of the hanging of Delhi parliament attack plotter Mohammed Afzal Guru.

Scores of security experts assisted by thousands of police personnel have been deployed. Bomb disposal squads have also been pressed into service while vehicular movement in the Kumbh area has been halted. More than a dozen hospitals have been set up to treat those who need medical aid.

VIPs and celebrities have been told to keep away from the congregation. "An appeal has been issued to all VIPs to either refrain from visiting the congregation on Sunday, or if they must do so, not to expect special security arrangements," said Kumbh Mela officer Mani Prasad Mishra.