A saffron-robed Hindu holy man was sworn in Sunday to lead India's most populous state, sealing what appears to be a shift in course by Prime Minister Narendra Modi that could redefine the world's largest democracy as a Hindu nation.

The choice of Yogi Adityanath, a firebrand Hindu ascetic with a history of agitation against minority Muslims, as Uttar Pradesh chief minister, stunned observers who said it marks a departure from the platform of development for all on which Modi rose to national power in 2014.

"Modi is saying India is a Hindu country and shall remain so," veteran journalist and commentator Shekhar Gupta said.