The party took place at Mumbai's Taj Lands End hotel in Bandra where the director filmed Abhishek Bachchan's wedding scene in the 2006 release.
Johar's Dharma Productions' offical Twitter page live tweeted the event on Friday (May 26).
"The beautiful Kajol is here!!!" posted the account, as the actress became one of the first guests to arrive. "Purab Kohli right behind her walks the red carpet.
"The multi-talented Ali Zafar in white suit just entered to rock the party."
Next to arrive was actor-turned-designer Shamita Shetty "in cute pink dress!".
Veteran stars Shabana Azmi, Javed Akhtar, Anupam Kher, Kiron Kher, Rishi Kapoor, Neetu Kapoor and director Yash Chopra were also in attendance.
"Anushka Sharma walks the red carpet gracefully," posted the production house.
Actresses including Kareena Kapoor, Karisma Kapoor, Rani Mukherjee, Genelia Deshmukh, Sonakshi Sinha, Malaika Arora Khan, Asin, Preity Zinta, Bipasha Basu, Urmila Matondkar and Deepika Padukone went to the party.
In spite of rumours that Johar had fallen out with Priyanka Chopra, the Dostana star was seen at the celebration.
"Look who just arrived!!! The most awaited couple Shah Rukh Khan and Gauri are just entering Karan's party!" added the Twitter handle.
Actors Hrithik Roshan, Farhan Akhtar, Riteish Deshmukh, Arjun Kapoor and Imran Khan turned up to congratulate the director.
Despite the party being described as a 'surprise', Johar is known to have collaborated with Manish Malhotra throughout the week on the interior design for the event.