"You are referring to a deranged person who gives statements for publicity," Kumar told reporters at the state Janata Dal (United) office.
He also criticised the Congress governments in Maharashtra and at the Centre for allowing Thackeray to "get away" with such remarks.
"It seems to me the Maharashtra government has outsourced governance to somebody else. We don't recognise this man. It is shameful the Maharashtra government has allowed somebody else to speak for it." Kumar said he did not want to disturb Prime Minister Manmohan Singh on the issue at this juncture.
"However, Biharis in Maharashtra should not worry, as it is entirely the responsibility of the Congress regimes in Maharashtra and at the Centre to provide them full security," he added.
Referring to Bihar's objection to Mumbai police conducting raids in the state without informing the local police, Kumar said such acts could cause law and order problems.
Thackeray had taken offence to Bihar's objection and had made the "infiltrator" remark on Friday.
A day later, Maharashtra home minister RR Patil had said Thackeray's speech had been recorded and would be examined.
"If there is anything objectionable, we will take relevant action against him," Patil had added.
Meanwhile, a case filed against Thackeray — after his threat — in a Bihar court is scheduled to be heard on Monday.