Prime Minister Narendra Modi's party is set to keep control of India's most populous state with a big majority, according to the count on Thursday of a state assembly vote that could offer clues to the national mood before a 2024 general election.

The Bharatiya Janata Party's (BJP) coalition was leading in about 250 of the 403 seats in Uttar Pradesh, news channels reported citing counting data, while the Election Commission said the party was ahead in enough seats to hold onto power.

"Everyone will be surprised by the mandate because we are going to win with a thumping majority," said Uttar Pradesh BJP spokesperson Rakesh Tripathi.

Uttar Pradesh is home to about a fifth of India's 1.35 billion people and sends the most legislators to parliament of any state.

The victory in the northern state has come despite the state and federal government's much-criticised handling of COVID-19, high unemployment, and anger over farm reforms that Modi cancelled last year after protests.

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The BJP has long predicted it would retain the state because of policies such as free staples for the poor during the pandemic, a crackdown on crime, and its popularity among the Hindu majority reinforced by the construction of a temple on the site of a razed mosque.

In elections in four smaller states over the past month, the Aam Aadmi Party that governs the national capital territory of Delhi is headed for a landslide victory in Punjab, while the races are tight but in favour of the BJP in Manipur, Goa and Uttarakhand.

Aam Aadmi Party leaders said they were ready to take on Modi nationally.

It has long been assumed in Indian politics that without winning Uttar Pradesh and the neighbouring state of Bihar, no party or coalition has much hope of securing a majority in parliament. The BJP has been in power in both.

For decades, Uttar Pradesh was a stronghold of the main opposition Congress party but it has been unable to stem a slide in its popularity over recent years.

A win in Uttar Pradesh would be a seal of approval for Hindu monk Yogi Adityanath, who was surprisingly chosen as chief minister for the state five year ago and is seen as a future prime ministerial candidate for the BJP.