Voters went to the polls in New Delhi on the first major day of India's marathon national election Thursday, with the capital a key battleground for a new anti-corruption party which shot to fame last year.

Almost a fifth of the parliament's 543 seats are up for grabs on Thursday, the third of nine phases of voting in the world's biggest election that will end when results are published on May 16.

As well as the capital and its 17-million residents, ballots were cast in densely populated rural constituencies in northern India where the Hindu nationalist frontrunner Bharatiya Janata Party (BJP) is expected to poll strongly.

But Thursday was of particular importance for the 18-month-old anti-corruption Aam Aadmi Party (AAP) which triumphed in the Delhi state election last December and is now contesting more than 400 parliamentary seats nationally.

Seen as a potential challenge to India's established political parties earlier this year, there were signs of disillusionment among some early voters after AAP's troubled time running the Delhi government.

The party has struggled to shake the "quitter" tag used by critics following the dramatic resignation of party chief Arvind Kejriwal just 49 days after he came to office as the capital's chief minister.

"We need stability. So I won't waste my vote on him," Jitender Singh, a 38-year-old rickshaw driver in a purple turban, told AFP in the old part of the city. "For now it is Modi, Modi, Modi for me. For the country actually."

"Polarising figure"

He was referring to BJP prime ministerial candidate Narendra Modi, the controversial hardline Hindu nationalist tipped to become prime minister at the head of a coalition led by his party.

His links to anti-Muslim riots in his home state of Gujarat and his uncompromising public statements make him a polarising figure, particularly for religious minorities.

But many voters have been swayed by his promises of economic development, strong leadership and clean government after a decade of rule by the scandal-tainted Congress party and the Gandhi political dynasty.

Kedarnath Agarwal, a 79-year-old speaking as shopkeepers rolled up their shutters and the first voters trickled into a nearby polling station, said he would abandon Congress for the first time in 50 years.

"All my hopes are pinned on Modi. He is a real leader, strong, decisive and experienced -- that's what we need this time," he said.

The 63-year-old politician made headlines Thursday after declaring for the first time that he was married, ending one of the biggest mysteries about his closely guarded private life.

Media reports had previously described how he walked away from a marriage arranged by his parents when he was a child, but this has never been confirmed by the man himself who has portrayed his single status as a virtue while campaigning.

"Riot victims"

In Uttar Pradesh state, a key battleground that sends 80 MPs to parliament, voters in an area hit by religious riots last August also went to the polls, including those still living in refugee camps.

The riots left more than 50, mostly Muslims, dead and tore apart communities in the district of Muzaffarnagar, with local politicians facing charges of inciting the violence.

The unrest is seen as having polarised the electorate in Uttar Pradesh along religious lines, with the BJP seen as benefiting from greater support from Hindus while secular-rooted parties promise to protect religious minorities.

"I will commit suicide, kill my children but not vote for Modi. He is so ferocious," Adisa Khatoon, a 35-year-old mother from one of the camps, told AFP.

The BJP has fielded two candidates from the area who have been charged with inciting the attacks which drove 50,000 people from their homes.

Elsewhere on Thursday, a second attack by Maoist rebels targeting security forces guarding pollings booths left two paramilitary policemen dead in eastern Bihar state, the Press Trust of India news agency reported.

On Wednesday, three policemen were killed in the insurgency-wracked central state of Chhattisgarh, parts of which were poll-bound on Thursday.

Back in Delhi, early voting took place peacefully and slowly after authorities declared a public holiday, with voters making their way to polling stations in bright spring sunshine.

Congress party chief Sonia Gandhi, dressed in a green sari, and her son Rahul, who is leading campaigning for a national election for the first time, cast their votes on Thursday morning.

The first two rounds of voting in India took place Monday and Wednesday in the remote northeast of the country where only 12 constituencies went to the polls.