Affectionately known as "Big B", he shot to stardom in the early 1970s in huge hit movies such as "Zanjeer" and "Sholay".
His films still open to packed cinemas across India, but his new movie -- comedy-drama "Gulabo Sitabo" -- was released online due to coronavirus restrictions closing cinemas.
Abhishek Bachchan, 44, is also a successful Bollywood actor and producer, and is married to actress and former Miss World Aishwarya Rai.
India's rising death toll
India's nationwide coronavirus toll rose on Saturday to 820,916 cases -- the third highest in the world -- with 22,123 deaths.
Health workers have complained about stretched facilities and severe staff shortages. Critics say the vast country is not testing enough and many infections are undiagnosed.
Prime minister Narendra Modi's government has eased lockdown restrictions despite a surge in cases, but he has warned the nation to be more cautious.
Bollywood recently resumed film shoots after a months-long hiatus due to the nationwide lockdown since late March.
Amitabh Bachchan is the doyen of India's movie industry, and his career has branched into television presenting, business and politics, as well as countless commercial endorsements.
Early in his acting life, he earned his reputation as India's "angry young man" for portraying violent heroes fighting an unjust system and injecting a new aggressive element into Bollywood movies, which had previously consisted of polite romances.
After some lean years, Bachchan bounced back spectacularly, largely due to his stint as host for the Indian version of the TV game show "Who Wants To Be A Millionaire?", which revived his artistic and financial fortunes.
According to local media, he was being treated at Mumbai's Nanavati hospital.
As best wishes poured in on Saturday, former Indian cricket star Sachin Tendulkar tweeted "praying for your good health and quick recovery."
The Mumbai region, which accounts for about a quarter of India's virus deaths, has suffered a new surge in infections, forcing authorities to build makeshift hospitals and quarantine facilities.