(CNN)India's ruling Hindu nationalist party has suffered a resounding defeat in state elections, boosting the opposition before national elections next year.
Official results show Prime Minister Narendra Modi's Bharatiya Janata Party (BJP) losing the three key states of Rajasthan, Madhya Pradesh and Chhattisgarh, with the opposition center-left Congress emerging as the single largest party in each.
Modi acknowledged defeat, saying in a tweet that "We accept the people's mandate with humility." It was the BJP's first major setback in state elections since it stormed to power nationally in 2014.
Congress, which had ruled India for most of the time since independence from Britain in 1947, was relishing an unexpectedly sizeable boost before a general election which must be held by next May. Results were still coming in but it appeared set to form the government in Chhattisgarh and could lead coalitions in Rajasthan and Madhya Pradesh.
"There is a huge responsibility on Congress party now and we will work towards fulfilling promises we made in these states," party president Rahul Gandhi told a press conference as jubilant supporters danced and set off firecrackers.
"A question is being asked about the future of our youngsters and how the country will create jobs. It is pretty clear that there is a feeling throughout the country that Mr. Modi hasn't delivered what he promised."
Poll results in two other states -- Telangana and Mizoram -- showed the BJP and Congress lost to regional parties respectively.
Analysts had predicted a setback for the BJP but the results "were worse than we had anticipated" (for it), according to Washington-based think tank Eurasia Group.
The results were a sharp reverse for Modi, whose party took 282 of 543 parliamentary seats in 2014.