Prince William and his wife Kate, as well as British Prime Minister David Cameron, signed a book of condolences at the French embassy and ambassador's residence today after the Paris attacks.
"To all those who have died and were injured in the heartless attacks in Paris, and to all the people in France: Nos plus sinceres condoleances (Our sincerest condolences)," William wrote at the embassy.
Cameron in his message expressed "shock, sadness and grief", signing the book at the west London mansion flanked by French ambassador Sylvie Bermann.
"We fly the French Tricolour as a symbol of our solidarity because we know our shared values -- liberte, egalite, fraternite -- will triumph over the evil faced in Paris last week," he added, underlining the word evil.
"Nous sommes tous ensemble (We are all together)" he wrote.
A series of coordinated gun and bomb attacks killed 129 people in the French capital late on Friday.
The Islamic State group claimed responsibility.
