A series of low-intensity bombs exploded on Sunday in the northeastern city of Patna, apparently targeting a vast rally featuring the opposition leader Narendra Modi, whose Bharatiya Janata Party hopes to unseat the long-dominant Congress Party in national elections next spring.
Homemade bombs, fitted with wires and timers, exploded in a series of crowded places: at a railway station, outside a movie theater, near two landmark buildings, and two on the Gandhi Maidan grounds, where Mr. Modi was preparing to speak, according to Abhayanand, the state's director general of police.
Four people were arrested Sunday afternoon, said Manu Maharaj, Patna's district police chief. He said that one man was arrested at the scene of a blast and confessed to being involved, and that all four men in custody were being interrogated.
Bharatiya Janata Party officials had hailed the rally as the largest to be held in the state of Bihar, a high-stakes electoral battleground, and decided to proceed despite the blasts. An enormous crowd roared in response as Mr. Modi invoked the Hindu epics, asking them to chant traditional battle cries.
Bihar has presented a problem for Mr. Modi's party. Commanding 40 seats in the lower house of Parliament, the state will be critical if the Bharatiya Janata Party hopes to win a comfortable majority.
But Bihar has a relatively large population of Muslim voters, many of whom are wary of Mr. Modi for his uncompromising stand in favor of his party's Hindu-nationalist ideology.
The state's top official, Nitish Kumar, broke off a longtime coalition with the Bharatiya Janata Party when it became clear that Mr. Modi was the probable candidate for prime minister.
In comments to reporters on Sunday, Mr. Kumar described the blasts as "a well-planned conspiracy to disturb and vitiate the peaceful atmosphere of the state," and appealed for harmony between India's political parties. He refused to speculate on the theory that the blasts were set by Indian Mujahedeen, a banned Islamic militant group, but did suggest they may have aimed to divide the state on religious lines.
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"If it was to disturb the communal peace and harmony of the state, we all have to take it and fight it out jointly," he said.
Mr. Modi, for his part, appeared calm and jovial at the rally, and the crowd erupted in cheers when he addressed them in Bhojpuri and Maithili, two local dialects. He said the Congress Party had failed miserably in its attempts to control inflation and unemployment, and contrasted his own humble background with the dynastic succession of the Nehru-Gandhi family.
"I used to sell tea on trains," Mr. Modi said. "Even the Railway Minister doesn't have my experience of what one faces on trains."
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It was a note that appeared to resonate with the crowd. Amit, who said he had come to the rally from a village three hours to the north, said Indians are "tired of dynastic rule."
"We have had the same family for 60 years," he said.
"We don't need this shehzada," he added, using the Hindi word for prince to refer to Rahul Gandhi, who is the son and grandson of previous Indian prime ministers and is widely expected to be the Congress Party's nominee for prime minister.
Throughout the day, speakers struggled to keep the crowd's attention as five separate bomb blasts went off. Each time, the official speaking did not acknowledge the explosion, awkwardly continuing speeches and trying to maintain the momentum of the rally.
The Bharatiya Janata Party spared no expense in the run up to the rally, trying alternative tactics to bolster awareness and attendance. In a nod to Mr. Modi's background as a tea seller, the party dispatched mobile tea vans throughout the city and gave "NaMo Tea Stall" posters to tea stands to brand themselves with the first two letters of Mr. Modi's first and last names.
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Mr. Modi's event in Patna, with the unfolding drama of the explosions, did manage to eclipse a simultaneous rally organized by the Congress Party near New Delhi, headlined by Mr. Gandhi.
The two events had been billed as "the battle of the rallies" by some journalists, but coverage favored Mr. Modi, and one observer noted on Twitter that she could not catch Mr. Gandhi's speech because news channels "had him on mute."