Mr Modi did not refer to the blasts in his speech, but later tweeted, "Blasts in Patna are deeply saddening & unfortunate."
Five people were killed and over 70 injured in seven crude bomb explosions in Patna before
Narendra Modi arrived at the city's Gandhi Maidan to address thousands of supporters and launch his party's campaign for the national election, due by May.
Mr Modi is the BJP's candidate for Prime Minister.
The Prime Minister has phoned chief minister Nitish Kumar and asked him to ensure a speedy investigation. The security lapse is a major ignominy for the chief minister, who ended his party's alliance with the BJP in August over its decision to give Mr Modi the starring role in its campaign for the national election. (
Modi targets Nitish at Patna rally)
Senior police officers said four people are being interrogated about the explosions.
BJP leader Sushma Swaraj described the blasts as "gross intelligence failure." Mr Modi did not refer directly to the blasts in his hour-long speech, but later tweeted, "Blasts in Patna are deeply saddening & unfortunate. Condolences with families of deceased & prayers with injured. I appeal for peace & calm.
The first of the seven blasts took place in a public toilet at the train station at about 10.30 am, less than three kilometres from the park where Mr Modi was scheduled to speak at 1 pm. A man who was injured there died after being hospitalized. (
Centre seeks report, rushes NIA team)
Then another four people were killed in six blasts which took place within 10 minutes at Gandhi Maidan. TV cameras showed smoke billowing and the crowd running. (
Caught on camera: bomb explodes at Modi's rally venue in Patna).
But the brief chaos ended by the time Mr Modi and other BJP leaders arrived on stage.
Today's rally was intended by the BJP as a blockbuster rebuttal to Mr Kuma who depends heavily on the support of Bihar's Muslims, and has questioned Modi's secular credentials.