"He was brought dead to the hospital. The cause of the death will be known after the post-mortem is conducted... it will take some time" Cooper Hospital Dean Dr Shailesh Mohite told news agency PTI.
According to initial media reports, Shukla died of a heart attack. But the hospital and Mumbai police issued statements saying that the cause of his death is not known yet.
"No injuries were sustained on the body of actor Siddharth Shukla. The cause of the death is yet to be ascertained. A team of police is present at the Shukla's residence for investigation," Mumbai Police told ANI.
The actor was best known for his role in the TV show 'Balika Vadhu'. He has also participated in several reality shows like 'Bigg Boss', 'Khatron ke Khiladi' and 'Jhalak Dikhla Jaa'.
Bollywood and daily soap personalities took to social media to pay tribute.
'Mirzapur' actor Manoj Bajpayee took to Twitter and wrote he was yet to come to terms with the actor's death.
"OMG! This is so shocking! Words will fail to describe the shock and sense of loss of his near and dear ones! May he rest in peace," he said.
Actor Himanshi Khurana, who was Shukla's co-contestant on "Bigg Boss" season 13, tweeted that it was "hard to believe" that the actor is no more.
Political analyst Tehseen Poonawalla, who also appeared as a contestant with Shukla on the reality show, wrote he was "speechless and heartbroken".
"We remained in touch post #BiggBoss13. Sending my deepest condolences to his mother who he loved so dearly. I love you #SidharthShukla& will miss you!" Poonawalla added.
Filmmaker Farah Khan paid tributes to the actor on Twitter.
"Can this year get any worse??! Shocked and extremely saddened to hear about @sidharth_shukla's demise. My heart goes out to his family," she wrote.
Rubina Dilaik, who was the winner of last year's 'Bigg Boss', on which Shukla made a brief appearance, posted a note on Instagram.
"Numb! May your soul rest in peace," the actor captioned the note, with a heartbreak emoticon.
Shukla began his career in showbiz as a model and made his acting debut with a lead role in the television show 'Babul Ka Aangann Chootey Na'.
He later appeared on shows such as 'Jaane Pehchaane Se… Ye Ajnabbi' and 'Love U Zindagi'.
Actor Riteish Deshmukh said he was "shocked beyond words".
"Gone too soon…. Condolences to his family, loved ones. - He was loved by millions. #SiddharthShukla you will be missed - rest in peace brother. Om Shanti." "Shocked and sad to know about Sidharth Shukla. Gone too soon. Prayers. Rest in peace," tweeted actor comedian Sunil Grover.