NEW DELHI: India Prime Minister Narendra Modi conceded defeat on Tuesday in Delhi state elections after vote counting showed anti-corruption campaigner Arvind Kejriwal's party heading for a landslide victory.

“Spoke to @ArvindKejriwal & congratulated him on the win. Assured him Centre's complete support in the development of Delhi,” Modi said in a tweet.

Read: 'Anarchist' vs 'policewoman': Delhiites come out to vote

BJP headed today for a major defeat in Delhi state elections at the hands of anti-corruption campaigner Arvind Kejriwal, in the premier's first loss since storming to power last year.

Officials began counting millions of ballots at 8:00am (local time), with exit polls from weekend elections predicting a huge loss for Modi's Hindu nationalist Bharatiya Janata Party (BJP).

Early tallies showed the BJP leading in just 12 seats, with former Delhi chief minister Kejriwal's Aam Aadmi Party (AAP) ahead in 55 in the 70-seat state assembly.

Hundreds of AAP supporters were massing outside the party's headquarters in Delhi, dancing, singing and waving flags with pictures of former tax official Kejriwal.

Also read: AAP tipped to defeat BJP in Delhi polls

Observers say a defeat will be a significant setback for a prime minister who has enjoyed an extended honeymoon with voters since his landslide general election victory in May.

A BJP spokesman said the results were not a sign of anti-Modi sentiment and that the election had been fought on local issues, including pledges for lower utility bills and countering corruption.

Also read: BJP rejects election defeat forecast in Delhi polls

“There is not an iota of negativity against the NDA (national) government anywhere in the country let alone in Delhi,” spokesman GVL Narasimha Rao told NDTV.

“There is a local election where local factors dominated."

A loss would set back Modi's efforts to consolidate power in the national parliament to push through promised economic reforms.

Modi needs to win state elections to gain control of both houses of parliament, where he is attempting to push through reforms on land acquisition, tax and other issues to revive the economy.

States are allocated seats in the nation's upper house where the BJP does not have a majority.