LONDON (AP) — Hundreds of British soldiers are taking part in the annual Trooping the Color ceremony in London marking Queen Elizabeth II's official birthday celebration in front of thousands of spectators.

The spectacle is expected to include an appearance by Prince Harry and his new wife, former American actress Meghan Markle, who will later join other members of the royal family on the balcony at Buckingham Palace after the event to watch a Royal Air Force fly by.

The 92-year-old queen, who recently had a successful cataract operation, watched the ceremony from a dais and will inspect the lines of guardsmen in bearskin hats and scarlet tunics.

The ceremony originated from traditional preparations for battle. Flags, or colors, were "trooped" so soldiers in the ranks would be able to recognize them.

The Queen's actual birthday is April 21.