An Indian court upheld on Thursday the result of an investigation that cleared Gujarat state chief minister Narendra Modi of complicity in riots in 2002, giving the opposition politician a boost as he runs for prime minister.

Modi, a leader of the Hindu nationalist Bharatiya Janata Party (BJP), has used the report to defend himself in the past, saying he was "given a thoroughly clean chit" and insisting he did all he could to stop the violence.

The riots erupted in February 2002 after a train carrying Hindu pilgrims was torched, prompting a wave of reprisal attacks against the state's minority Muslims. At least 1,000 people were killed, most of them Muslims, in some of India's worst religious bloodshed since 1947.

A special team appointed by the Supreme Court to investigate the role of Modi and 62 other people in the violence said in a 541-page report in 2012 it could find no evidence to prosecute the chief minister.

Most importantly, it cleared Modi of the most damaging allegation: that he had told senior officials to allow Hindu mobs to vent their anger.

Zakia Jafri, the widow of a politician belonging to the ruling Congress party who was killed by rioters along with dozens of neighbours, had filed a protest petition against the team's report in April.

On Thursday, a court in Gujarat's main city of Ahmedabad rejected Jafri's petition, saying there was no evidence to prosecute Modi.

"Truth alone triumphs," Modi wrote on his Twitter page.

Jafri's lawyers and supporters said they would take the case to a higher court within a month.

"Modi can feel easy for 20 days but not for more than that," said Mihir Desai, a lawyer for Jafri.

Modi, who in the years following the riots turned Gujarat into one of India's fastest growing states, has built a reputation as a business-savvy and investor-friendly administrator. But he has been unable to fully shake off allegations over the riots.

"I strongly feel for those who haven't got justice in Gujarat," Law Minister Kapil Sibal said soon after the court decision on Thursday.

Modi looks to be the front-runner in an election that must be held by May.