Britain's Queen Elizabeth led an armada of 1,000 boats in a gilded royal barge down London's River Thames in a spectacular highlight of four days of nationwide celebrations to mark her 60th year on the throne.
Hundreds of thousands of people armed with "Union Jack" flags and wearing red, white and blue clothes and hats, poured into the capital from early morning, braving wind and rain to line the 11km route of one of the largest flotillas ever seen on the river.
Up and down the country, millions more were due to attend diamond jubilee street parties over the long holiday weekend in honour of the 86-year-old queen, the only British monarch after Queen Victoria to have sat on the throne for 60 years.
"With the economy the way it is, this is why we have to have this celebration. It gives people a lift," said Kevin Rogers, 54, who travelled across southern England with his wife and son to bag a viewing spot at London's Albert Bridge.
Organisers say today's pageant will be the largest of its kind in 350 years since a similar spectacle was held for King Charles II and his consort Catherine of Braganza in 1662.
A floating belfry with a set of eight church bells specially cast for the celebrations will be at the head, with bells from riverbank churches pealing out as it passes by.
The queen will be on board "The Spirit of Chartwell" barge with her 90-year-old husband Prince Philip and other members of the royal family including heir-to-the-throne Prince Charles, his eldest son Prince William and new wife Catherine.
The flotilla will travel under 14 bridges and past 14 miles of bunting. Another of the boats, Amazon, also featured in diamond jubilee celebrations for Queen Victoria, Elizabeth's great-great-grandmother, in 1897 when Britain's empire spanned much of the globe.
Although the queen is still head of state in 16 countries from Australia and Canada to tiny Tuvalu in the Pacific Ocean, Britain is now a shadow of the world power which once ruled over more than a third of the planet.
Historians and commentators say the jubilee and the pomp and pageantry synonymous with British royal occasions gives the country a sense of national pride at a time when the economy is in recession and people face deep austerity measures.
The government also hopes it will kick-off a summer of revelry capped off by the Olympic Games in London which will raise the public's spirits and their own poll ratings.
"One of the great things that a monarch brings and, particularly a royal family and her majesty the queen personally brings, is this sense of national unity and stability - someone who the whole country can identify with," David Cameron, Britian's prime minister said in a broadcast today.
The celebrations come as polls show the vast majority of Britons support the monarchy, which has overcome a slump in the 1990s following marital infidelities and the death of the hugely popular Princess Diana in a 1997 Paris car crash.
Celebrations will continue on Monday with a pop concert outside Elizabeth's London residence Buckingham Palace, where Paul McCartney and Stevie Wonder will be among the acts. Madness are set to take to the roof of the famous landmark to belt out hit song "Our House".
The long weekend concludes with a service of thanksgiving at St Paul's Cathedral on Tuesday followed by a carriage procession along the broad Mall leading to Buckingham Palace where the queen will wave to the crowds from the balcony.
For the police, Sunday's flotilla presents a new security challenge and acknowledge that attention-seeking stunts could disrupt big set-piece events.
"We've had officers searching under the water, on the water, in the air and on the land," Stephen Kavanagh, London police's deputy assistant commissioner, told Reuters news agency.