Prime Minister Narendra Modi's party has won a landslide victory in India's most important battleground state in a personal triumph that will strengthen his claim to a second term as national leader.
Wresting control of Uttar Pradesh is a ringing endorsement of Modi's stewardship of Asia's third-largest economy after his high-risk decision to scrap high-value banknotes worth 86 per cent of the cash in circulation.
The Election Commission of India said Modi's Bharatiya Janata Party had won a clear majority on Saturday, according to partial results.
The BJP was on course to win 309 of 403 seats in the state assembly, the biggest majority for any party in the state since 1980.
Almost four in 10 voters backed Mr Modi's party, the election commission said as it tallied the last votes, 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.
"I give my heartfelt thanks to the people of Uttar Pradesh. This is a historic victory for the BJP; a victory for development and good governance," Mr Modi told his 28 million followers on Twitter.
Investors hope victory will embolden Mr Modi to embark on more reforms, including the launch of a national sales tax, to boost economic growth.
"The outcome of this election will enable Mr Modi to sharpen his winning anti-corruption and improved governance policies as he begins to position himself for the 2019 general election," wrote analysts Shailesh Kumar and Sasha Riser-Kositsky of macro advisory firm Eurasia Group.
Mr Modi threw himself into the Uttar Pradesh campaign after his party got off to a slow start, addressing dozens of rallies and turning the contest into a test of his popularity and his radical move to abolish big banknotes to rein in corruption.
Celebrations erupted outside BJP offices in state capital Lucknow and Delhi, with party workers dancing and splashing each other with paint in keeping with the Holi festival of colours that Hindus are celebrating this weekend.