Rivals say the ruling Bharatiya Janata Party is whipping up religious tensions to win the election in Bihar, the country's third most populous state and critical to Modi's plans to improve his party's strength in the upper house of parliament.
Modi's administration has faced a rising tide of criticism for failing to rein in hardline Hindu groups that are campaigning for issues such as cow worship in a multi-faith country.
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But Modi, addressing a political rally in Purnea, sidestepped the criticism and accused his long-time foe, the Congress party, of playing partisan politics.
Re-opening a sensitive chapter in recent history, Modi said that hundreds of Sikhs were massacred in Delhi by supporters of the Congress soon after the assassination of prime minister Indira Gandhi by her Sikh bodyguards in November 1984.
"Should the Congress be giving lessons in tolerance?" he said to thunderous applause from supporters wearing saffron headbands.
"Sikh families in Delhi are still grieving. Have you tried to wipe their tears." Modi himself has faced allegations that he looked the other way or was complicit in attacks on Muslims in Gujarat state that he governed after a mob set a train carrying Hindu pilgrims on fire.
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He has denied the charges and a Supreme Court ordered-investigation absolved him of any wrongdoing. Hindus make up around 80 percent of India's 1.2 billion population, followed by Muslims, Sikhs and Christians.
Since the BJP swept to power, Hindu groups have stepped up a campaign against intermarriage with Muslims as "love jihad", begun rewriting school textbooks to reflect a nationalist view and are demanding a ban on cow slaughter.
Last month a Muslim man was beaten to death by a mob for eating beef and another was murdered for espousing atheism, stoking fears for India's tradition of tolerance.
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"There is extreme intolerance," Bollywood superstar Shah Rukh Khan told India Today television channel, the latest artist to speak up after a string of writers returned awards given by the government in protest against attacks on religious minorities.
Central Bank governor Raghuram Rajan also called for greater tolerance.
Modi didn't respond to those comments, but promised voters that he would lift Bihar out of poverty. Control of Bihar, which lies next to most populous Uttar Pradesh, is key to Modi's ambition to secure a majority in the upper house of parliament.
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