Sir Ian McKellen, who plays Gandalf the wizard in the blockbuster, led the round of applause and cheers for the Duchess on Wednesday (12Dec12) and asked her husband Prince William to pass on their best wishes.
The Duchess cancelled her appearance in Leicester Square as she is suffering from hyperemesis gravidarum, which leaves expectant mothers feeling extremely nauseous in the early months of pregnancy.
The Prince told the crowds gathered, "She would have loved to have been here if she could."
Meanwhile Australian actress Cate Blanchett, who plays elf queen Galadriel, thanked fans for braving freezing temperatures for the occasion.
She said, "I'm cold. I'm sure everybody is. The weather is enough to drive anyone away, but it is great to see so many fans."