"Lost my elder brother Razak bhai due to cardiac arrest. Please pray for his soul," the actor's close friend Shehzad Khan posted on Facebook, while confirming the death.
Khan, 65, was rushed to Holy Family hospital in suburban Bandra at around 12.30 AM where on arrival doctors declared him dead.
His funeral is likely to be held tomorrow at the Narialwadi Kabaristan at Byculla in the city.
Filmmaker Priyadarshan, who worked with Razak in "Hera Pheri" and "Hungama", mourned the death of the comedian.
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In Salman Khan-starrer "Hello Brother", Khan played Ninja Chacha, which is one of his most remembered roles. He also starred alongside Shah Rukh Khan in "Baadshah" and Aamir Khan in "Raja Hindustani".
Khan's other notable roles include "Anari No 1", "Har Dil Jo Pyar Karega", "Joru Ka Ghulam", "Aamdani Atthani Kharcha Rupaiyaa" and "Bhagam Bhag". His last film appearance was "Kyaa Kool Hain Hum 3" this year.
Actor Rishi Kapoor wrote, "R I P. Razak Khan,a colleague of many a film. Condolences to all concerned!"
"Funny man Razak Khan has also passed away. May his soul rest in peace," Arshad Warsi posted.
Filmmaker Hansal Mehta wrote, "Sad to hear about passing away of Razzak Khan. His finest performance in 'Dubai Return' with @irrfan_k unfortunately remains in the cans."
Comedian Rajpal Yadav said the actor will be missed.
Kunal Kemmu, who co-starred with Khan in "Raja Hindustani" as a child actor, said, "Shocked and saddened to hear this. Always smiling and making everyone laugh. You will be missed RIP #RazakKhan."