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The 73-year-old thespian, who has given hits like "Sholay", "Deewar" and "Piku", shared the news on his Twitter handle as well as his official blog on Sunday.
He tweeted: "21 million followers...thank you all for this...25 million here we come! baaaadduuummmmbaaa."
On his blog, he posted: “It is 21 million on Twitter and there is great jubilation among the family of the blog and the extended family.”
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The "Paa" star added: "I feel elated. It's like an intoxication and one that you can breath through. So to all that who helped need to be rewarded by a remembrance ... thank you all."
The cinema icon is ahead of superstars like Shah Rukh Khan, who has 19.6 million followers, Salman Khan with 17.8 million, Aamir Khan with 17.6 million and Priyanka Chopra, who has 14 million followers, on the micro-blogging website.
On the work front, he will be next seen in "Te3N" and "Pink".
IANS