Fisher had been on a tour to promote her new book, The Princess Diarist
Star Wars actress Carrie Fisher has suffered a massive heart attack on a flight from London to LA, according to US media reports.
TMZ is reporting that doctors who happened to be on board performed life-saving CPR on the 60-year-old star, who is best-known for playing Princess Leia in Star Wars.
The website said the emergency occurred 15 minutes before the plane landed in LA.
After landing paramedics reportedly rushed her to a nearby hospital.
Actress Anna Akana was on board the flight and tweeted that Fisher stopped breathing "for 10 minutes or so".
"Don't know how else to process this but Carrie Fisher stopped breathing on the flight home," she wrote. "Hope she's gonna be OK.
"So many thanks to the United flight crew who jumped into action, and the awesome doctor and nurse passengers who helped."
A spokeswoman for police at LAX airport said: "Los Angeles police responded at 12.15pm to Terminal 7 regarding a female victim who went into cardiac arrest.
"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."
Peter Mayhew, who is best known for playing Chewbacca, tweeted: "Thoughts and prayers for our friend and everyone's favourite princess right now.. @carrieffisher."
Fisher had been on a tour to promote her new book, The Princess Diarist.
She was once married to the singer Paul Simon and has a daughter from another relationship, actress Billie Catherine Lourd.