By Jatindra Dash and Frank Jack Daniel

BHUBANESWAR/NEW DELHI, India (Reuters) - An Islamist militant group is believed to be behind an attack on a rally by Indian Hindu nationalist politician Narendra Modi that killed six people and wounded more than 80, police said on Tuesday.

Modi, who has a good chance of becoming India's next prime minister, is seen as a target of militants who hold him responsible for riots a decade ago during his first term as chief minister of Gujarat state. At least 1,000 people, most of them Muslims, were killed in the rioting.

Seven crude bombs went off on Sunday in Bihar state as Modi's supporters gathered for his rally. He was not near any of the blasts and delivered his speech despite the attack.

Senior police official S.N. Pradhan said one of two suspects arrested after the blasts had identified a suspected senior member of the Indian Mujahideen militant group, Tehseen Akhtar, as the organizer of the attack.

The National Investigation Agency, India's top counter terrorism body, is seeking the arrest of the 24-year-old Akhtar in connection with attacks in recent years in the cities of Mumbai and Varanasi and is investigating his role in blasts in Hyderabad city in February.

"Because of the Tehseen connection, the entire chain is established," said Pradhan, a senior police official in Jharkhand state, where the detained suspect is from.

"There is no doubt that it is the work of the Indian Mujahideen."

Police said the bombers were trying to spark a stampede in the crowd. Heavy loss of life could have inflamed tension, recently simmering again, between majority Hindus and minority Muslims.

The Indian Mujahideen has been accused of dozens of similar bomb attacks over recent years.

The ease with which the bombs were planted around the rally ground, where tens of thousands of people gathered to hear Modi, has raised concern about Modi's safety.

Leaders of his Bharatiya Janata Party (BJP) have called the attack a big security failure and have demanded better protection for him.

There has been no claim of responsibility.

LEADER ARRESTED

Opinion polls suggest the BJP, which is seeking to unseat a ruling coalition led by the Congress party, could win the most seats in a general election due by May.

Critics say Modi did not do enough to stop rampaging Hindu mobs in the 2002 riots in Gujarat. Modi denies any role in the violence and a Supreme Court appointed panel cleared him of wrongdoing.

Modi rejects any suggestion of bias against Muslims although his party rose to prominence after leading a campaign that led to the destruction of the Babri Masjid mosque in 1992, which triggered rioting that killed some 2,000 people.

The party said the mosque was built on a Hindu holy site.

The Indian Mujahideen claimed responsibility for a series of bomb attacks in Gujarat in 2011 that killed at least 45 people, saying they were revenge for the 2002 riots.

India arrested the organization's suspected leader, Yasin Bhaktal, near the border with Nepal in August but Sunday's explosions would appear to indicate the group can still launch high-profile attacks.

Police also suspect the Indian Mujahideen's involvement in a series of blasts in July at the Buddhist holy site of Bodh Gaya, which is also in Bihar state.

A National Intelligence Agency official told Reuters an analysis of the bombs used in the Sunday attack showed they were similar to devices used in Bodh Gaya. In both attacks, ammonium nitrate was detected, and the timers and circuits were similar, the official said.

In 2011, the United States designated the Indian Mujahideen a terrorist organization with close links to Pakistan-based groups like the Lashkar-e-Taiba (LeT).

The State Department said the group played a "facilitative role" in the 2008 Mumbai attack carried out by LeT that killed 166 people, including six Americans.

(Writing by Frank Jack Daniel; Editing by Robert Birsel)