Known for his roles in films like King Kong and Faulad, Singh was admitted to Kokilaben hospital last evening and since then has been in a critical condition.
"He is extremely critical and on ventilator. He is on a high dose of medication to support his blood pressure. We hope to see some improvement through the day," Dr Ram Narain, Chief Operating Officer at Kokilaben hospital, told PTI.
"But we will continue to keep a watch on him," he said.
Yesterday, the 83-year-old actor's son Vindu had said that his father was serious.
"He is serious. There is blood clot in his brain or he has suffered heart attack... We are not sure what has happened. We have been told that he is under observation for 24 hours," Vindu said.
The veteran actor played the role of Hanuman in Ramanand Sagar's television series Ramayan and was last seen in Imtiaz Ali's Jab We Met in 2007 where he acted as Kareena Kapoor's grandfather.