Thackeray, 86, is in a "very critical" condition, doctors treating the Sena leader told PTI.
As the news spread, several party workers assembled outside his residence, 'Matoshree', in suburban Bandra, where security has been stepped up.
Thackeray, who is ailing for sometime, continues to be under constant medical supervision.
A team of doctors stationed inside 'Matoshree' with nursing staff has been taking care of the Sena patriarch, who is having breathing problems.
Last month, he could not attend his party's annual Dussehra rally in Mumbai, but addressed it through a video recorded message.
Several leaders, including former President Pratibha Patil, have visited Thackeray's residence over the past few days to enquire about his health.