Tamil Nadu Chief Minister J. Jayalalithaa suffered a cardiac arrest on Sunday evening and is being treated by a team of specialists at Apollo Hospitals, the hospital said.
In a statement issued here, the hospital said: "She is being treated and monitored by a team of doctors including cardiologists, pulmonologists and critical care specialists."
It is learnt Governor C. Vidyasagar Rao is returning here on Sunday night from Mumbai.
The evening saw hectic activity at Apollo Hospitals with senior officials and others assembling there.
Some of the AIADMK cadre assembled outside the hospital broke down on hearing about the news and were fervently praying for her well-being.
The 68-year-old AIADMK supremo was admitted to the hospital for treatment of fever and dehydration on September 22.
The doctors later said she needed a longer hospital stay as she was suffering from infection, and put her on respiratory support.
According to Apollo Hospitals, a team of specialist doctors -- cardiologists, respiratory physicians, consultants for infectious diseases, diabetologist and endocrinologist -- treated her.
Recently Apollo Hospitals Chairman Pratap C. Reddy told media that Jayalalithaa is returning to normalcy and all her vital organs are functioning well.
She was shifted out of Intensive Care Unit as there were no health complications, he had said.
Reddy also said Jayalalithaa can go home whenever she decides to do so.