LONDON, May 6 (Reuters) - Charles III will be crowned king on Saturday in Britain's biggest ceremonial event for seven decades, a sumptuous display of pageantry dating back 1,000 years.Charles succeeded his mother Queen Elizabeth when she died last September and at 74, he will become the oldest British monarch to have the 360-year-old St Edward's Crown placed on his head as he sits upon a 14th century throne at London's Westminster Abbey.

Watched by about 100 heads of state and dignitaries including U.S. first lady Jill Biden, Charles follows 40 predecessors in being crowned at the abbey - which has staged all the country's coronations since William the Conqueror back in 1066.

His second wife Camilla, 75, will also be crowned queen during the two-hour ceremony which, while rooted in history, will attempt to present a forward-looking monarchy and nation.

"No other country could put on such a dazzling display - the processions, the pageantry, the ceremonies, and street parties," Prime Minister Rishi Sunak said.

"It's a proud expression of our history, culture, and traditions. A vivid demonstration of the modern character of our country. And a cherished ritual through which a new era is born."

Despite Sunak's enthusiasm, the coronation is taking place amid a cost of living crisis and public scepticism, particularly among the young, about the role and relevance of the monarchy and questions about its finances.

Saturday's event will be on a smaller scale than that staged for Queen Elizabeth in 1953, but will still aim to be spectacular, featuring an array of historical regalia from golden orbs and bejewelled swords to a sceptre holding the world's largest colourless cut diamond.

After the service, Charles and Camilla will depart in the four-tonne Gold State Coach that was built for George III, the last king of Britain's American colonies, riding back to Buckingham Palace in a one-mile procession of 4,000 military personnel from 39 nations in ceremonial uniforms.