The 83-year-old actor is said to be in a critical condition, reports the Hindustan Times.
"He is serious. There is blood clot in his brain or he has suffered heart attack... we are not sure what has happened," said the star's son Vindu Dara Singh.
"We have been told that he is under observation for 24 hours," he went on to add.
Also a professional wrestler, Singh is reported to have started the trend of male actors going shirtless in Indian films.
The actor later won acclaim for his work in television, playing the role of Indian god Hanuman in the TV adaptation of the Ramayan.
Singh's most recent film role was playing Kareena Kapoor's grandfather in Imtiaz Ali's 2007 release Jab We Met.