NEW DELHI: India hosts its biggest day of voting Thursday in its marathon election, with the Nehru-Gandhi dynasty battling to save the ruling Congress party from defeat to opposition Hindu nationalist leader Narendra Modi.

More than 195 million voters in 121 constituencies across a dozen states are eligible to cast ballots in the largest single day of polling in the over month-long election which ends May 12.

The key day comes after India's most famous family cranked up the rhetoric in a bid to claw back support from Modi, saying the Bharatiya Janata Party (BJP) candidate was a threat to the nation's secular foundations.

The Nehru-Gandhi clan, which has dominated the country since independence from Britain in 1947, has emerged fighting with Congress president Sonia Gandhi, daughter Priyanka and son Rahul all on the campaign trail.

Surveys have suggested Congress, under Rahul's leadership, is heading for a crushing defeat at the hands of the right-wing BJP.

Sonia, 67, the left-leaning Congress party matriarch, told voters Modi would rip India's secular fabric, accusing him of representing a “dangerous combination of religious fanaticism, power and money”.

Sonia's daughter, Priyanka, 42, seen as the most gifted politician of India's so-called “first family” which has given the country three premiers, said Modi would spread divisions between the majority Hindu and minority Muslim communities.

“The ideology of Congress is to unite people and maintain the unity of the country while that of the opposition is to divide,” she said.

India 'first family' feud

Priyanka has strongly taken up the torch for India's secular identity, accusing her estranged cousin Varun Gandhi, a BJP candidate, of “betrayal” of the family heritage in a headline-making feud this week.

Rahul, 43, has accused Modi of displaying “anger and arrogance”.

Modi's Hindu nationalist rhetoric and failure to swiftly curb 2002 deadly anti-Muslim riots that swept Gujarat state when he was chief minister have stirred worries among critics about his ability to maintain secular peace.

The BJP leader has never been found guilty by official investigations of any wrongdoing over the riots in which at least 1,000 people, mainly Muslims, died.

Modi, 63, struck back late Wednesday at the Gandhi family's attacks, saying it was their “obsession to pull Modi down”.

“But Modi does not lose, does not die,” he said, referring to himself in the third person, according to local media.

He rejected demands he apologise for the riots, saying he committed no crime.

“If I am guilty, hang me in the street square,” he said.

He added Muslims in the holy Hindu city of Varanasi where he is seeking a parliamentary seat “love me when they meet me”.

Modi, who has declared only he can revive India's sagging economy, said the BJP would score its “best” victory while the “useless and corrupt” Congress faced its “worst” defeat.

An opinion poll by NDTV network this week forecast the BJP would win 226 seats and with its coalition allies grab over 50 percent of the constituencies.

Voters have turned against Congress over massive graft scandals, spiralling inflation and the sharp economic slowdown, polls indicate.