Explosions at an election rally killed five people and wounded 83 in Patna on Sunday, Bihar chief minister r Nitish Kumar said.
At least six crude bombs exploded in quick succession near a crowd of tens of thousands of people waiting to hear a speech by opposition candidate Narendra Modi for the election due by May.
Earlier, a small device detonated at Patna's train station.
"There was a blast right behind me. I heard people saying that one of the tyres of a vehicle has burst," one witness told a TV network. "When I went to the spot, I saw many people lying injured on the ground."
Television channels showed pictures of shattered windows of buses, white smoke billowing and people running in panic.
Kumar said five people were killed and 83 were wounded in the bombings.
The attacks were a reminder of modern India's history of political violence, from the assassination of leaders such as Mahatma Gandhi to a separatist militancy in Kashmir and a present-day Maoist guerrilla uprising.
Blasts at big political rallies are unusual and the attack was the first of its kind since Modi, from the Hindu nationalist Bharatiya Janata Party, began a campaign to oust the Congress party-led coalition that has governed India for a decade.
It was not immediately clear who was behind the attacks and no group claimed responsibility. A senior police officer in Patna told Reuters one person had been arrested and had confessed to planting a bomb at the train station. Police detained four other suspects for questioning, he said.
Police are investigating the possible involvement of the Indian Mujahideen, a home-grown Islamist group that has carried out low-level bomb attacks in cities across India in recent years, the officer said, asking not to be named.
"We should be able to get a clearer picture by tomorrow," the officer said.
Modi's critics view him as a polarising figure on whose watch Muslims were attacked in deadly rioting in 2002 in Gujarat which he has governed for three straight terms.
But to his growing army of supporters, including many in Indian business, the BJP strongman is the best hope the nation has of turning around the economy after he helped turn Gujarat into one of the country's fastest-growing states.
Modi himself has denied any allegations of bias against India's minority Muslim population. A Supreme Court-appointed panel cleared him of any wrongdoing in the riots.
On Sunday, he sought to broaden his appeal, saying Hindus and Muslims both needed to be lifted out of poverty.
"Do you want to fight against poverty or against Hindus?," he asked. "Poor Muslims have to fight against poverty. Poor Hindus have to fight against poverty."
Modi did not mention the blasts in his speech, but later called them "deeply saddening and unfortunate" on Twitter. He appealed for peace and calm.
The first explosion went off in a public toilet at Patna's main train station where Modi supporters were streaming in on special trains for the rally.
Then, just before he arrived, small bombs went off at different places near the sprawling public park, sowing panic. At least three unexploded bombs were found near the venue, said Sushil M Khopde, inspector general of police in Patna.
Such bombings of political rallies are rare in India. In 1991, a Tamil suicide bomber killed former Prime Minister Rajiv Gandhi during a campaign rally in southern Tamil Nadu state.
His son Rahul Gandhi, now leading the campaign for the Congress party and the party's most likely candidate for prime minister, said this week he feared he could be killed, like both his father and grandmother, former Prime Minister Indira Gandhi.
Rahul Gandhi and Modi are among the most heavily guarded figures in the election.
Sunday's rally was intended as a show of strength in Bihar where the governing party broke off a 17-year alliance with the BJP after it picked Modi as its candidate to lead the party.
The state sends 40 lawmakers to parliament's lower house and could prove decisive in forming a new government.
The BJP has picked up support since making Modi its candidate for prime minister last month, but would need allies to form a government, two new opinion polls showed this month.