Singh was admitted to Kokilaben Dhirubhai Ambani Hospital & Medical Research Institute in Mumbai on Saturday evening.
Doctors attending on him on Wednesday said Singh's Magnetic resonance imaging (MRI) scan has showed significant damage to his brain.
"Dara Singh continues to be very critical. His brain has been damaged significantly due to decreased oxygen supply to brain," said Dr Ram Narain, Chief Operating Officer of the hospital on Wednesday.
Meanwhile, Singh's close friends from the film industry, including Dharmendra, Raza Murad and Farah Khan visited him at the hospital.
Singh's son Vindu said he needs prayers of his well-wishers now.
"Daraji will be kept under observation for at least 24 hours. Though he is being treated by great doctors, prayers are needed," Vindu told TOI.
Meanwhile, Singh's native village Dharmoochak near Amritsar is praying for his recovery.
Residents of the village are praying at a Hanuman temple for him since Sunday.
"The news is not good but we are hopeful and are praying to Lord Hanuman, he will be well soon," said Dara Singh's nephew Baljit Singh Pehelwan was quoted as saying by TOI.
Dara Singh, who was born in 1928 in Dharmoochak in Amritsar district of Punjab began as a wrestler but soon became a popular actor in Indian cinema and played several roles opposite yesteryear's actresses like Mumtaz in the 1960s.
He played in films like King Kong, which were inspired by Hollywood movies.
He was nominated member of Rajya Sabha by Bhartiya Janata Party during Aug 2003 – Aug 2009.
He had a successful career in television also and was a popular face in many of the TV serials since the 1980s.
But with the role of Hanuman in Ramayan on TV, he reached every household.
His films in the 1960s included titles like Toofan (1969), Jawan Mard (1966), Thakur Jarnail Singh (1966), Boxer (1965), Hum Sab Ustad Hain (1965), Khakaan (1965), Lutera (1965), Mahabharat (1965, played Bheem), Raaka (1965), Rustom-E-Hind (1965), Sher Dil (1965), Sikandar E Azam (1965) , Tarzan and King Kong (1965), Tarzan Comes to Delhi (1965), Teen Sardar (1965), Aandhi Aur Toofan (1964), Aaya Toofan (1964), Badshah (1964), Darasingh: Ironman (1964), Dara Singh Hercules (1964), Samson (1964), Veer Bhimsen (1964), Faulad (1963), Rustom-E-Baghdad (1963) and King Kong (1962).
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