Karnataka Chief Minister S M Krishna on Thursday resigned as the Congress faced the prospect of a defeat in the assembly polls.
He submitted his resignation to Governor T N Chaturvedi at Raj Bhavan.
Earlier, Karnataka Pradesh Congress Committee president B Janardhan Poojary said the results were "unexpected".
"We accept the people's verdict," he said, noting, however, that the anti-incumbency wave was not as severe as in Andhra Pradesh.
Asked if the Congress would join hands with the Janata Dal-Secular to make up the required numbers to form the government, he said the party high command would take a decision on the issue.
He, however, asserted, "We will not allow BJP to rule the state at any cost."