India's opposition Congress party won power in a key state Saturday, partial election results showed, defeating Prime Minister Narendra Modi's ruling BJP a year ahead of national polls.

It ousted Modi's Bharatiya Janata Party from office in Karnataka, the only southern state controlled by the Hindu nationalist grouping.

Karnataka has a population of well over 60 million people -- about the same as Britain -- and its capital Bengaluru is India's tech hub.

With dozens of results still to come in, Congress had already won 114 places in the 224-seat assembly, enough for an overall majority, and was leading in another 22, which would give it a comfortable cushion, the election commission website showed.

BJP state leader B.S. Yediyurappa -- a former chief minister -- conceded defeat.

"Victory and defeat aren't new to BJP," he told reporters. "We will introspect about the party's setback. I respectfully accept this verdict."

The party had mounted a major campaign in the state with Modi himself visiting to promote its muscular brand of Hindu politics.

At one of his rallies, Modi praised an incendiary new film that wildly exaggerates the number of Hindu women converting to Islam and joining the Islamic State jihadist group.

Modi -- who is widely expected to stand again in the 2024 general election -- also attempted to woo Hindu voters by chanting an ode to the monkey god Hanuman.

Congress campaigned hard on secularism, giveaways of electricity and rice for the poor, and accusations of BJP corruption.