Dozens of others were injured in the explosions Sunday in the eastern Indian state's capital of Patna.
The attacks began with an explosion at the city's railway station lavatory and spread closer to large grounds near the station where Narendra Modi -- widely expected to be the prime ministerial candidate of India's main opposition Bharatiya Janta party (BJP) in next year's general elections -- later went to address a mammoth rally as concerns rose across the country about growing political violence in the world's largest democracy in advance of elections.
Authorities reported seven explosions.
Unexploded bombs were found near the venue of the rally and defused.
Though no group had claimed responsibility for the attacks, some Indian media reports said police were investigating the role of an Indian Islamic militant organization.
The blasts injured more than 80 people, some critically, authorities said.
Eastern Bihar state is ruled by Janata Dal (United) party, led by Chief Minister Nitish Kumar, whose relations with the BJP have soured after their parties broke off their alliance.
The Indian Express reported four people had been arrested, including one police said was battling for life at a Patna hospital after allegedly planting the bomb at the railway station. Police said they had seized suspicious documents from the suspects.
The Express said police had not officially termed the blasts terror attacks but the city's police chief, Abhay Anand, confirmed the use of improvised explosive devices and timers.
The NDTV television channel reported police were investigating the role of the Indian Mujahedin, described as an Islamisc militant group blamed for a number of recent deadly bombings in the country.
The TV channel quoted police as saying they had major leads from one of the suspects and that they believe the mastermind of the attack is one who had been allegedly trying to set up an Indian Mujahedin module by recruiting boys from a neighboring state.
The report said police, using information provided by the suspect, raided his village and said they found incriminating evidence.
"We found explosive devices, literature, pen drives and cash. There was evidence of his involvement in illicit activities," a senior Bihar police official was quoted as saying.
Police said they believe the blasts were designed to create panic and set off a stampede at Modi's rally, which would have killed or injured more people.
Speaking at news conference, Kumar said the investigation was being handled by the federal government's National Investigating Agency.
Modi's party has blamed Kumar's government for security lapses.
The blasts are a frightening turn in the political campaign for the general elections next year as various parties vie for power. The current federal coalition government is led by the Congress Party of Prime Minister Manmohan Singh.
The report said Modi's party decided to go ahead with the rally despite the blasts and that party leaders feared calling it off could to panic and a stampede.