TV footage showed several small explosions and smoke outside the venue in Patna, in the state of Bihar, where several hundred thousand people were gathering for the rally.[break]
Footage showed people running from the low-intensity explosions.
The rally later went ahead with other leaders of the opposition Bharatiya Janata Party (BJP) taking to the stage ahead of Modi.
Another small explosion also occurred at a railway station in Patna, killing one person, a local police officer said.
"One person succumbed to his injuries after a bomb explosion at platform number 10 of Patna railway station," local police official Manu Maharaj told AFP on the phone.
Maharaj confirmed the four other blasts near the venue, and said police were making several arrests.
Hindu nationalist Modi was named last month as the BJP´s prime ministerial candidate for national elections due by May next year.
The chief minister of economically successful western Gujarat state, Modi is popular with the corporate world, with many hoping he can turn revive Asia´s third-largest economy, if elected next year.
But he remains a divisive figure, tarred by the religious riots in Gujarat in 2002 in which as many as 2,000 people were killed, mainly Muslims, according to rights groups.
Modi was chief minister at the time and denied any wrongdoing, but one of his former ministers was jailed last year for orchestrating some of the violence.
The Supreme Court once likened him to Nero, the emperor who fiddled while Rome burned.
Modi and leaders of the ruling Congress party are holding a series of mass rallies across the country in a battle to win five key state elections later this year.
Those elections are seen as a crucial test of popularity, with both parties hoping to capitalise on any momentum from the results for next year´s elections.
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