"Lalu Prasad is a rental player of Congress," Kumar said addressing an election rally at Madhubani in support of JD(U) candidate Gulam Gaus.
Considering the "dark" past of RJD, nobody could trust him, Kumar said taking a jibe at his political rival who has been claiming support of the people for RJD and Congress in the Lok Sabha polls in Bihar.
It took eight years to undo the damage done to Bihar under RJD rule and bring the state on the path of development, he said.
"Will anyone like to return to those dark days when people could not venture out of home after sunset" he asked the audience and highlighted change in the state where people could roam freely even at night.
Prasad has been making rounds of different constituencies in the state to seek votes for RJD-Congress alliance candidates.
Kumar also came down heavily on his old ally BJP over hate speeches made by its leaders in order to "communally polarise" voters.
The Bihar CM highlighted developmental works done by his government and sought people's support for its continuation.