Sharad Yadav (75) died at a private hospital in Gurugram on Thursday.
The Bihar government declared state mourning on Friday as a mark of respect to veteran socialist leader and former union minister Sharad Yadav.
The chief minister's office issued the statement declaring the state mourning.
Chief Minister Nitish Kumar condoled Yadav's death saying, "I had a very deep relationship with Sharad Yadav Ji. I am shocked and saddened by the news of his death. It has caused irreparable loss in social and political fields.
"He (Yadav) was a strong socialist leader, a seven-term Lok Sabha MP and a three-term Rajya Sabha MP. May his soul rest in peace,” he added.
Nitish Kumar also spoke to Sharad Yadav's family.
Sharad Yadav(75) died at a private hospital in Gurugram on Thursday night after he was brought to the medical establishment in an unconscious and unresponsive state. He had collapsed at his Chhatarpur residence.
Governor Phagu Chauhan and the RJD chief Lalu Prasad Yadav, recuperating in Singapore after a kidney transplant surgery, too condoled the death of the socialist leader.
Lalu Prasad, who fought electoral battles with Sharad Yadav in Madhepura Lok Sabha seat, said that mutual differences with him "never resulted in any type of bitterness".
Mr Prasad in a video statement from his hospital bed referred to Sharad Yadav as 'bade bhai' (big brother) and recalled his old association with him.
"Sharad Yadav, besides late Mulayam Singh Yadav, Nitish Kumar and I learnt the politics of socialism from Ram Manohar Lohia and Karpoori Thakur," Prasad said.
"On many occasions, Sharad Yadav and I fought with each other. But our disagreements never led to any bitterness," said the RJD supremo, who had lent a helping hand to Yadav when the latter was struggling to come out of political wilderness.