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 (BJP), 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.

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.

Election authorities on Wednesday issued an order rebuking Amit Shah, who runs the BJP's campaign in Uttar Pradesh, India's most populous state and a key political battleground, over his speeches.

Reuters