No casualty was reported, they said.
Six fire tenders and five water tankers were rushed to the site to contain the blaze, a senior fire brigade official said.
Though some episodes of the reality show were shot at the studio, there was no shoot planned for today, sources said.
"The show is shot at R K Studio, but there was no shoot planned for today as the channel had already shot a couple of episodes. The next shoot is planned for the first week of October. We are hopeful that we will be able to make amends by that time and get the set ready.
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P S Rahangdale, chief of the Mumbai fire brigade, said the fire was reported around 2.20 pm.
"The blaze was brought under control by evening. The cooling operations are still on. There is no report of anyone suffering injury in the fire so far," he said.
Thick and dark smoke enveloped the entire area as flames swept through the basement of the sprawling studio, where cinema legend Raj Kapoor, his siblings and children made a number of films over the decades.
Another fire brigade official confirmed that the ground floor of the studio was gutted in the blaze.
Expressing sadness over the incident, actor Rishi Kapoor, son of Raj Kapoor, tweeted, "Sad a major fire broke out at RK Studio. We have lost the iconic Stage 1. Thankfully no casualties nor injuries. All your concerns appreciated."
The iconic studio was founded in 1948. The films made under the RK banner include "Aag", "Barsaat", "Awaara", "Shri 420", "Jis Desh Mein Ganga Behti Hai", "Mera Naam Joker", "Bobby", "Satyam Shivam Sundaram", "Ram Teri Ganga Maili", among others. The last film made under the banner was "Aa Ab Laut Chalen".
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