By Agence France-Presse
The 60-year-old "Star Wars" actress was flying from London to Los Angeles when she suffered cardiac arrest, and was given cardiopulmonary resuscitation by an emergency responder on board.
Fisher collapsed 15 minutes before the plane landed at LAX, according to celebrity news website TMZ, and was rushed to UCLA Medical Center on a ventilator.
TMZ, citing witnesses, said the actress's eyes were closed and she appeared unconscious as she was rushed through the terminal, where paramedics worked for 15 minutes before they could get a pulse.
The Los Angeles Times said her condition was critical, quoting an unnamed source who said the actress was "in a lot of distress on the flight."
The Los Angeles Fire Department (LAFD) did not refer to the actress by name but confirmed it had responded to an alert just after midday over "a patient on an inbound flight in cardiac arrest."
"LAFD firefighter paramedics were standing by and provided immediate advanced life support and aggressively treated and transported the patient to a local hospital," spokesman Erik Scott told AFP.
United Airlines said medical personnel met Flight 935 from London upon arrival after the crew reported that a passenger was unresponsive.
Fisher was catapulted to worldwide stardom as the rebel warrior Princess Leia in the original "Star Wars" trilogy, which has been a cultural phenomenon since the release of the films from 1977 to 1983.
Steeped in Hollywood excess from an early age, she was the product of the four-year marriage of movie star Debbie Reynolds, best-known for her role in "Singin' In The Rain," and singer Eddie Fisher.
The relationship, and the happy home in Beverly Hills, came to an end when Fisher left Reynolds for her close friend, the actress Elizabeth Taylor.
- Searingly honest -
Fisher's co-stars led tributes as Hollywood reacted with shock to news of her collapse.
"As if 2016 couldn't get any worse... sending all our love to @carrieffisher" tweeted Mark Hamill, who plays her on-screen twin Luke Skywalker in the "Star Wars" saga.
The American actress has talked and written frequently about her years of drug addiction and mental illness.
Fisher is known for her searingly honest semi-autobiographical novels, including her best-selling debut "Postcards from the Edge" which she turned into a film of the same name in 1990.
She has given various interviews over the years about her diagnosis of bipolar disorder and addiction to prescription drugs and cocaine, which she admitted using on the set of "The Empire Strikes Back" (1980).
She has also discussed being treated with electroconvulsive therapy, in which small electric currents are passed through the brain, to trigger brief seizures.
Writer and actress Anna Akana, who said she was on board Fisher's flight, described the mid-air drama in a series of tweets, voicing her "shock and sadness."
"Don't know how else to process this but Carrie Fisher stopped breathing on the flight home. Hope she's gonna be OK," she said.
"So many thanks to the United flight crew who jumped into action, and the awesome doctor and nurse passengers who helped. Feel weird even tweeting about it but I JUST finished her book and was fangirling out over seeing her dog Gary in person."
Fisher's famous "Star Wars" character features as part of the storyline to spin-off "Rogue One," which is currently riding high in box offices around the globe, although the actress is understood not to have been involved in the production.
"The whole world is sending you so much love! Sending you the universe's most powerful Force," tweeted English actress Gwendoline Christie, who played the warrior Brienne of Tarth in HBO's "Game of Thrones" and Captain Phasma in Fisher's most recent "Star Wars" episode, "The Force Awakens."