India held the biggest day of its mammoth general election on Thursday, with a quarter of its 815-million-strong electorate eligible to vote during a week of fresh blows for the ruling Congress party and gains for the Hindu nationalist opposition.

Narendra Modi, the prime-ministerial candidate of the opposition Bharatiya Janata Party, has been wooing voters with promises to rescue India from its slowest economic growth in a decade and create jobs for its booming young population.

In the latest large opinion poll, the BJP and its allies were forecast to win a narrow majority in the 543-seat lower house of Parliament, compared to previous surveys predicting that they would fall short.

Yet a decision by the Election Commission to reprimand a senior Modi aide for making speeches deemed to stir tensions with minority Muslims underlined critics' assertions that the party is a divisive force.

Voting took place in 120 constituencies across 12 states, from the fractious Himalayan region of Jammu and Kashmir - where election materials had to be airlifted to some remote polling stations - to the lush southern state of Karnataka, whose capital is the IT and outsourcing hub Bangalore.

The world's biggest-ever election is taking place in nine stages from April 7 to May 12, with results due on May 16.

"We want Modi to win this time. That's why we are here early in the morning, doing our best for him," said Preetham Prabhu, a 32-year-old software engineer who was the first to cast his vote in a polling station in a residential suburb of Bangalore.

Modi's image remains tarnished by Hindu-Muslim riots in Gujarat, the western state where he is chief minister, on his watch 12 years ago. More than 1,000 people, most of them Muslims, were killed in the violence.

Modi denies accusations that he failed to stop the riots, and a Supreme Court inquiry found he had no case to answer. In an interview with ANI television news on Wednesday, Modi accused reporters of smearing him over the riots.

The Congress party, led by the Nehru-Gandhi dynasty, is forecast to suffer its worst-ever defeat after a decade in power due to public anger over the economic slowdown, high inflation and a string of graft scandals. The party has ruled India for more than 50 of its 67 years of independence.

Congress has struggled in recent days with a former media adviser and a former coal secretary both releasing books that paint Prime Minister Manmohan Singh as a well-intentioned but weak figure who answers only to party President Sonia Gandhi.

"It's only a dynasty, like previously we had kings ruling," said P.V. Padmanabhan, a 79-year-old retired electricity board official who has voted in every Indian election, and was lining up to vote at the eastern Bangalore polling station.

"They have to give it to somebody else. (Leaders) should not only come from Nehru's family," referring to Jawaharlal Nehru, India's first prime minister.

Indian elections are notoriously hard to forecast due to the country's diverse electorate and parliamentary system in which local candidates hold great sway.

Opinion polls wrongly predicted a victory for a BJP-led alliance in elections in 2004 and underestimated Congress' winning margin in 2009.

Thursday's parliamentary candidates include IT billionaire and Infosys co-founder Nandan Nilekani, running for Congress in Bangalore, and Maneka Gandhi, an estranged member of the Nehru-Gandhi dynasty standing for the BJP in Uttar Pradesh.

Voter turnout has been 68 percent on average in the 111 constituencies that have voted so far, according to the Election Commission, a sharp increase in the 60 percent in the same constituencies and 58 percent nationally in 2009.

"It is because of the people's unrest against the establishment. It is the anti-incumbency," Nitin Gadkari, a BJP leader and the party's former president, told Reuters.

A voter is helped at a polling station in Kamshet, a village in the western Indian state of Maharashtra, on Thursday. About 815 million people have registered to vote in the world's biggest election - a number exceeding the population of Europe, and a world record. Results of the mammoth exercise, which ends on May 12, are due on May 16. Danish Siddiqui / Reuters