In the midst of a grand celebration, the British royals were out in full force for the Diamond Jubilee River Pageant in London

In the midst of a grand celebration, the British royals were out in full force for the Diamond Jubilee River Pageant in London, England on Sunday (June 3).

Held as Britain is marking 60 years on the throne for Queen Elizabeth II, the festivities saw a military uniform clad Prince William alongside wife Kate Middleton - who was radiant in a red Alexander McQueen frock with a matching hat, tan heels and a clutch bag.

With one million people estimated to have gathered along the shoreline to see the extravaganza, fellow royal family members joining in on the waterbound outing included Prince Harry, Prince Phillip, Prince Charles and Camilla.

Once everyone was on deck and situated, the Royal Barge launched from Albert Bridge's pier while following behind the Gloriana rowing vessel manned by 18 oarsmen - after which 1,000 steam boats and tugs, speed boats and historic vessels ended up taking to the Thames River when all was said and done.

Duchess of Cambridge resplendent in red Alexander McQueen as she boards Royal barge for Queen's Diamond Jubilee river pageant

 The Duchess of Cambridge cut a swathe through the gloomy weather this afternoon as she stepped on to the Spirit of Chartwell wearing a vibrant scarlet Alexander McQueen dress. 

Topping off the bright outfit with a flamboyant red hat by Sylvia Fletcher from royal milliner James Lock & Co, Kate made sure the public lining the banks of the Thames would be able to spot her easily aboard the barge.

Fletcher designed the Duchess's distinctive red maple leaf hat worn on her tour of Canada last summer.