The defeat came after Modi won an overwhelming victory in last year's national elections, with the current poll seen as a verdict on the Prime Minister's economic program.
A spokesman for the Hindu-led nationalist party has, meanwhile, rejected speculation that the election result is a personal blow for Modi.
Prime Minister Narendra Modi's party appears to have been routed in state assembly elections in Bihar, third most populous state in India.— David Barstow (@DavidBarstow) 8 ноября 2015
According to the spokesman, the BJP "managed a creditable performance."
Photo: Voters queue outside booth in Mahua, India, to vote in parliamentary elections in Bihar state — ibnlive pic.twitter.com/YchyyJTCmM— SavageNation (@SavageNation) 28 октября 2015
According to the latest reports on the election results, the anti-Modi alliance is ahead in 125 seats, compared with the BJP's 79 seats, with votes still being counted.
Located in India's north-west, Bihar has a population of 100 million people and is believed to be one of the country's largest and poorest states.