India began voting on Friday in a six-week election with an all but assured victory for Hindu nationalist Prime Minister Narendra Modi, as a weakened opposition is pushed to the sidelines.

A total of 968 million people are eligible to take part in the world's biggest vote – a staggering logistical exercise that critics say follows a concerted effort to delegitimise rivals.

A long and winding queue was patiently assembled outside a polling station in the Hindu holy city of Haridwar, on the banks of the Ganges river, even before the booths opened.

"I am here because I am happy about the direction the country is headed," said autorickshaw driver Ganga Singh, 27. "I will vote keeping in mind not personal welfare but the country's prosperity."

Modi, 73, remains resoundingly popular after a decade in office that has seen India rise as an economic power.

"I urge all those voting... to exercise their franchise in record numbers," he wrote in a social media post on X as the election began. "Every vote counts and every voice matters!"

Modi has already led the ruling Bharatiya Janata Party through two landslide victories in 2014 and 2019, forged in large part by his appeals to the Hindu faithful.

His prospects have been further bolstered by several criminal probes into his opponents and a tax investigation this year that froze the bank accounts of Congress, India's largest opposition party.

Opposition figures and human rights organisations have accused Modi's government of orchestrating the probes to weaken rivals. (AFP)