Ailing Shiv Sena chief Bal Thackeray's condition was described as "critical" early Thursday, top party sources said.
His son and party executive president Uddhav Thackeray made an appeal to all party activists to "remain calm" and said treatment was continuing on the ailing leader.
Thackeray, 86, has been ill for over three weeks, causing great concern in political circles.
Mumbai Police and other security agencies deployed tight security outside 'Matoshri', the clan's home in Bandra west.
All roads leading to 'Matoshri' were also barricaded and even some party leaders had to abandon their vehicles near the Western Express Highway and walk the short route to the Thackeray household.
Thousands of Shiv Sainiks have arrived here from Mumbai and surrounding areas as news of Thackeray's deteriorating health spread Wednesday night.
"Thackeray is serious, but stable. A team of doctors is continuously monitoring him," a senior party official who met him this evening, told IANS, preferring anonymity.
However, the medical team is yet to take a decision about when and whether to put Thackeray on a ventilator or not.
Besides all members of the Thackeray family, nephew Raj Thackeray and his wife Sharmila, top Shiv Sena leaders like ex-Lok Sabha Speaker Manohar Joshi, senior leader Subhash Desai, party MPs Sanjay Raut, Ramdas Kadam, senior Bharatiya Janata Party leader Vinod Tawade and others rushed to the bungalow around midnight.
An anxious Bollywood mega-star Amitabh Bachchan accompanied by son Abhishek also went to 'Matoshri' even as people awaited the latest update on Thackeray's delicate health condition.