More than 1,000 boats are sailing down the Thames in a flotilla tribute to Queen Elizabeth II's 60 years on the throne.
Hundreds of people ignored the persistent rain and camped out overnight to secure prime riverside spots. Crowds swelled into the thousands on June 3 along the 11 kilometer route.
The Thames barrier is being lowered to slow the river's flow.
The nautical parade promises to be the most spectacular in London for some 350 years.
Among the flotilla vessels will more than three dozen "Dunkirk little ships," private boats that rescued thousands of British soldiers from the beaches of France after the German invasion in 1940 -- a defeat that became a major victory for wartime morale.
The four-day Diamond Jubilee celebrations also included thousands of street parties across the country on June 3.
Prince Charles and his wife, Camilla, the duchess of Cornwall, joined hundreds of people for a damp al fresco lunch on Piccadilly, one of London's main shopping streets.
The celebrations also include thousands of street parties and a pop concert on June 4 in front of Buckingham Palace featuring Elton John and Paul McCartney.
The celebration kicked off on June 2 with a royal day at the races, as the queen watched a horse with the courtly name of Camelot win the Epsom Derby.
The 86-year-old monarch and her husband, Prince Philip, visited Epsom racecourse south of the capital for the derby, one of the year's biggest horse-racing events.
The queen is a racing fan and horse breeder who has attended the derby for decades and reads the "Racing Post" each day over breakfast although, unlike many of her subjects, she does not gamble.
Jubilee events end on June 5 with a religious service at St. Paul's Cathedral, a carriage procession through the streets of London, and the queen's appearance with her children, grandchildren, and great-grandchildren on the palace balcony.
The queen took the throne in 1952 on the death of her father, King George VI, and most Britons have known no other monarch.
But not everyone in Britain is celebrating.
The antimonarchist group Republic held a riverbank protest June 3 to oppose the wave of jubilee-mania.
With reporting by AP, AFP, and Reuters