Narendra Modi's party has won a landslide victory in one of India's key states, a boost for the prime minister halfway through his first term.

The Bharatiya Janata Party (BJP) took control of northern Uttar Pradesh state - India's most populous - with the biggest majority there since 1980.

Mr Modi personally led the campaign against regional rivals the Samajwadi Party and Bahujan Samaj party.

Trends suggest the BJP is also set to win the northern state of Uttarakhand.

The Election Commission of India said the Hindu nationalist BJP had won a clear majority in Uttar Pradesh, with nearly four in 10 voters backing the party.

"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 wrote on Twitter.

The result will strengthen the BJP in the upper house of India's parliament, where its reform efforts have been hampered by the lack of a majority.

Mr Modi has been central to his party's election strategy. He campaigned aggressively on a promise to bring growth and modernisation, and to root out corruption.

These were strong promises in an impoverished state like Uttar Pradesh where caste, family and religious affiliations are deeply entrenched.

Mr Modi also strongly backed his move to ban high-value notes - amounting to 86% of India's currency - last year as a measure to tackle corruption.

The incumbent chief minister, Samajwadi Party leader Akhilesh Yadav, opposed the currency move, and told the electorate that Mr Modi had "taken money out of people's pockets", and hurt businesses.

He also told people that he was the best person to bring development.

"My track record in the past five years shows that I have a vision for the state," he said at a rally recently.

Analysts say Mr Yadav's decision to form an alliance with the Congress party, and his direct attack on Mr Modi's policies probably hurt his prospects.

Congress, the main opposition party, remains in the lead in Punjab and Manipur states.

Victory for 'adopted son' of Uttar Pradesh - Vikas Pandey, BBC News, Delhi

The BJP's resounding win in Uttar Pradesh has proven that Mr Modi's ability to connect with the voter has not diminished.

The party did not declare a chief ministerial candidate, and relied heavily on Mr Modi.

The opposition called him "an outsider from Gujarat state", and criticised the BJP for not having a local leader, but this has clearly not affected sentiment on the ground.

The prime minister was able to successfully project himself as the "adopted son of Uttar Pradesh". He is an elected MP from the state.

The victory in Uttar Pradesh, as well as the neighbouring state of Uttarakhand will further boost his stature in national politics.

But there will be challenges ahead. People have put faith in Mr Modi's leadership, but he is not going to rule the state.