Prime Minister Narendra Modi's party was on course for a landslide victory in India's most important battleground state, early returns showed on Saturday, in a personal triumph that will strengthen his claim to a second term as national leader.

Wresting control of Uttar Pradesh would be a ringing endorsement of Modi's stewardship of Asia's third-largest economy after his high-risk decision last November to scrap high-value banknotes worth 86 percent of the cash in circulation.

Though premature to call the final outcome, the Election Commission of India said Modi's Bharatiya Janata Party (BJP) was leading in 301 of the 403 seats in the northern state, which if confirmed by results would give it the biggest majority scored by any party in the state since 1980.

Almost four in ten voters backed the BJP based on early counting, the election commission said, close to the party's vote share in Uttar Pradesh in the 2014 national election when it won the biggest national majority in three decades.