Ford and Fisher appeared together as Han Solo and Princess Leia in four Star Wars blockbusters, and Fisher recently revealed they had a brief love affair decades ago.
"I'm shocked and saddened to hear the news about my dear friend," Ford told The Hollywood Reporter Saturday. "Our thoughts are with Carrie, her family and friends."
"As if 2016 couldn't get any worse... sending all our love to @carrieffisher," their co-star Mark Hamill -- who plays Luke Skywalker in the franchise -- tweeted.
"On this Christmas Eve join me in sending prayers & light to our Princess. Carrie, The world is rooting for you my friend. We love you!" Peter Mayhew, who plays Chewbacca in the Star Wars universe, posted Saturday.
"I don't pray much anymore but I am praying for you right now, @carrieffisher. Please pull through..." offered filmmaker Kevin Smith, who directed Fisher and Hamill in his big-screen comedy Jay and Silent Bob Strike Back.
Fisher's brother Todd has said the 60-year-old actress is in the intensive care unit of a California hospital. She fell ill on a flight from London to Los Angeles and an EMT passenger administered CPR until the plane landed and she could be transported to the hospital.
as if 2016 couldn't get any worse... sending all our love to @carrieffisher— Mark Hamill (@HamillHimself) December 23, 2016