Carrie Fisher has been stabilised and is in intensive care following a medical emergency on a plane last night (December 23).
Fisher was in London promoting her recent book The Princess Diarist which collects her journals from her days working on Star Wars.
Fellow travellers on board the flight identified the Star Wars actress as the passenger who had fallen ill, tweeting their shock.
Carrie Fisher Extended InterviewActress Carrie Fisher talks about Wishful Drinking, the one-woman show she created and performs that's soon headed to Broadway. The actress got a heart attack 15 minutes before her flight landed in Los Angeles from London.
Actress Carrie Fisher speaks onstage during Wizard World Comic Con Chicago 2016 at Donald E. Stephens Convention Center on August 21, 2016 in Rosemont, Illinois.
Peter Mayhew, 72, who plays the legendary Chewbacca in the Star Wars franchise said: 'Thoughts and prayers for our friend and everyone's favorite princess right now'.
"I JUST finished her book and was fangirling out over seeing her dog Gary in person", Akana wrote.
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Fisher is alive, but she is in critical condition, the site said.
"Upon arrival the LAFD (Los Angeles Fire Department) was already performing CPR on the victim and the victim was transported to a local hospital for further medical treatment".
Fisher's representative did not immediately respond to a request for comment.
A few passengers, who were on Carrie's flight, also took to social media to share what they saw.
'I ask everyone to stop for a moment and send special thoughts to Carrie Fisher, ' he noted on Twitter.
The actress, who is the daughter of singer Eddie Fisher and singer/dancer/actress Debbie Reynolds, is also known for roles in Blues Brothers and Soapdish.
A large gathering of media personnel was camped outside Ronald Reagan UCLA Medical Center in Los Angeles hospital, where TMZ and the Los Angeles Times reported she had been taken.
"He's incredibly private; I feel really bad for doing that to him."