Shiv Sena chief Bal Thackeray, who has been ailing for weeks, was in "critical" condition late Wednesday night.
The 86-year-old has been put on a life support system, media reports said, adding that he was not able to breathe on his own.
Thackeray's kidney functions have also failed.
Earlier on Tuesday, dismissing reports of his serious health situation, Thackeray said urged people to ´throw out´ Congress governments both from the Centre and the state of Maharashtra.
"Throw out Congress and drive away the darkness forever," Thackeray said in a statement published in Sena mouthpiece ´Saamna´.
Thackeray made the comments a day after he dismissed reports that he was on ventilator support and said he has not collapsed completely.
He said his son and executive president of Shiv Sena Uddhav Thackeray was looking after him while his nephew and Maharashtra Navnirman Sena (MNS) chief Raj Thackeray ´keeps coming regularly´.