NEW DELHI (AFP) - India hosted its biggest day of voting on 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.

Voters lined up at 7am in 121 constituencies across a dozen states where more than 195 million voters are eligible to cast ballots in the largest single day of polling in the five-week election, which ends on May 12.

"The polling has started at 7 am on a peaceful note," said Umesh Sinha chief electoral officer of key battleground state Uttar Pradesh.

Elsewhere voters cast ballots for the first time in the southern city of Bangalore, home to IT giants Microsoft and Google, which is a key battleground between the Congress and opposition Bharatiya Janata Party (BJP).

In central Chhattisgarh, where an attack by Maoist insurgents left 14 dead at the weekend, the second phase of voting on Thursday was marked by a new bomb attack on a railway line in the Bokaro district which disrupted travel.

The key polling day comes after India's most famous family cranked up the rhetoric in a bid to claw back support from Modi, saying the 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 Mr Gandhi's leadership, is heading for a crushing defeat at the hands of the right-wing BJP.

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

Ms Priyanka Gandhi, 42, seen as the most gifted politician of India's so-called "first family" which has given the country three premiers, said Mr 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.

Ms Gandhi 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.

Mr Gandhi, 43, has accused Mr Modi of displaying "anger and arrogance".

Mr 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.

Mr Modi, 63, struck back late on 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 that 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".

Mr 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.

Premier Manmohan Singh, 81, a respected economist handpicked by Mrs Gandhi after she returned Congress to power following a nearly decade-long stint in the political wilderness, has been portrayed in a new book by his former press aide as a powerless puppet.

Congress said the decision to wheel out Mrs Gandhi and Ms Gandhi, who easily outshines her more awkward brother Rahul, was part of an earlier mapped strategy but analysts say it smacks of desperation.