More than 10 million devout Hindus seeking absolution from their sins took a dip in holy waters in northern India during a span of four hours on Wednesday, authorities said, as they braced for millions more to swarm the site of the Kumbh Mela.

Authorities stepped up the numbers of police officers and put air ambulances on standby in the city of Prayagraj in Uttar Pradesh state on one of the holiest days of the Hindu festival, considered the world’s largest gathering of humanity.

“There are more people in this city in one day than the population of many countries, and the numbers are swelling by the minute,” Prashant Kumar, the state’s chief of police, told Reuters.

The numbers arriving had pushed infrastructure arrangements to the brink, making delays and traffic jams inevitable, he added. Media said vehicles were backed up for hundreds of kilometers from the edges of the city.

More than 10 million people had bathed by 8am, authorities said, with more expected.

The full moon on Wednesday, known as Magh Poornima, makes it one of the holiest days in the six-week-long festival, held at the confluence of India’s three holiest rivers.

Maintaining safety can prove a challenge at the festival, despite stringent precautions.

More than 30 people were killed in a stampede on Jan 29, officials said, as over 76 million flocked to the river for a ‘royal dip’, but did not deter a stream of notables, from India’s president to ministers, film stars and the wealthy.

Hindus believe that a plunge in the waters where the Ganga, Yamuna and the mythical Saraswati meet will absolve them of sin but they consider this year’s event even more significant as having the power to free them of the cycle of rebirth.