EXCLUSIVE

3:58 PM PT -- Carrie's brother, Todd, just told AP she's in stable condition.

2:46 PM PT -- We're told when the plane landed paramedics worked on Carrie for 15 minutes with CPR before they were able to get a pulse. She's currently at UCLA Medical Center on a ventilator.

Carrie Fisher suffered a massive heart attack Friday on an airplane ... TMZ has learned.

Our sources say Carrie was on a United flight from London to LAX when she went into cardiac arrest. People on board were administering CPR. United Airlines says its crew reported Carrie as "unresponsive" when they landed.

We're told the emergency occurred 15 minutes before the plane landed in L.A. A flight attendant asked if there were any medical personnel on board and an EMT who was sitting in the back of the plane came up to first class and administered life-saving measures.

Witnesses at LAX tell us Carrie's eyes were closed and she appeared unconscious as EMTs rushed her through the terminal. She had an oxygen mask on her face.

The plane landed just after noon in L.A. and paramedics rushed her to a nearby hospital.

Fisher, who became famous, of course, as Princess Leia from 'Star Wars,' has been on a book tour.