"All are talking about the Delhi gang-rape, but nobody is asking where these men came from. No one is asking who did this. No one is talking about the fact that all these rapists are from Bihar," Mr Thackeray said in Mumbai on Saturday evening.
He said while he is abused whenever he raises the issue of Bihari criminals, even Delhi Chief Minister Sheila Dikshit has commented on the matter. "She (Dikshit) said in the presence of the Prime Minister and Congress president Sonia Gandhi that migrants in Delhi were responsible for the spate of criminal activities," Mr Thackeray said.
Five persons have been arrested and charged with the rape and murder of the medical student. They have been summoned in court on Monday. The sixth suspect has claimed he is a minor. If proved so, he will be tried under laws for juveniles.
Mr Thackeray and his party are known to make provocative statements against north Indian migrants and accuse them of infiltrating Mumbai and taking away jobs that they say rightly belong to Marathis.
The MNS chief's latest remark didn't go unnoticed by political leaders in Bihar. State Animal Husbandry Minister Giriraj Singh of the Bharatiya Janata Party said Mr Thackeray had lost his "mental balance" and demanded a ban on the MNS.
Rashtriya Janata Dal chief Lalu Prasad Yadav said Mr Thackeray has a "habit of speaking in this manner" and action should be taken against him.
Janata Dal-United leader Ali Anwar said Mr Thackeray "has gone mad" and challenged him to make such statements outside Maharashtra.
In Maharashtra, the Congress pointed to the MNS's own crime record. "Last month, an MNS leader was arrested for raping a minor girl. Three years ago, a minor domestic help was gang-raped by four MNS workers. An MNS leader was arrested for the rape of a girl in Amravati in April 2011," said state Congress spokesperson Sachin Sawant.
"Raj Thackeray is bereft of issues and is resorting to such statements to divert attention from his partymen's actions," he said.