Modi is campaigning on a platform to revive an economy growing at its slowest in a decade and end the red tape and corruption that have bedevilled the Congress-led coalition.
The Congress fared badly inelections in four large states towards the end of last year, largely due to voters' anger over corruption.
Compared with the relatively untested Gandhi, Modi has years of experience as the chief minister of the western state of Gujarat where he has built a reputation as a business-savvy and investor-friendly administrator.
But he has been unable to fully shake off allegations over anti-Muslim riots in Gujarat in 2002 in which at least 1,000 people were killed, most of them Muslims.
Modi denies wrong doing and a Supreme Court investigation found no evidence to prosecute him.
Singh was scathing about Modi, saying he would be bad for India.
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"I sincerely believe that it would be disastrous for the country to have Narendra Modi as the prime minister," said the softly-spoken Singh.
Singh rejected a suggestion that he had been a weak prime minister and again referred to Modi.