The Queen's Diamond Jubilee weekend will reach its peak later on Sunday with a colourful River Thames pageant.
Hundreds of thousands of people will line the banks of the river to watch the Queen cruise past on a royal barge escorted by a 1,000-strong flotilla.
The event to mark 60 years of her reign promises to be the most spectacular nautical event in London for 350 years.
Millions more Britons are expected to take part in Jubilee parties, outdoor concerts and fairs across the country.
The Queen, Duke of Edinburgh and other senior royals will travel from Albert Bridge to Tower Bridge aboard a lavishly adapted royal barge - the Spirit of Chartwell - decorated with almost 1,000 cut flowers.
Dunkirk small ships
As the pageant begins at 11:30 BST with mustering at Hammersmith and Battersea bridges, the Thames barrier will be lowered to slow the river's flow.
The spectacle, along a seven-mile stretch, will end some six hours later when the last boat passes Tower Bridge.
Adam Kerr, the skipper of a restored 19th Century Cornish fishing lugger, the Barnabas, sailed 400 nautical miles to take part.
"It's going to be wonderful, I think it's going to be a pretty colourful spectacle," he told the BBC.
"I'm sure they're very worried about the security of boats crashing into each other, I think we'll be okay - we're a pretty well controlled boat, good crew and nice engine to drive us along."
The £10m cost of the event has been met by private donations but the security costs will be paid for by the taxpayer.
The anti-monarchy group, Republic, has said it will hold a demonstration against what they call an unelected, unaccountable monarch.
BBC weather forecaster John Hammond said pageant watchers should wrap up with waterproofs and wind proofs amid temperatures of around 11C.
He said party-goers in most of England and Wales should expect cloud and the threat of rain, whilst Scotland and Northern Ireland would be dry with some sunshine.
A collection of small ships used to rescue stranded troops from the beaches of Dunkirk in 1940 will also take part in the river event.
It will be led by the Motor Torpedo boat 102, the flagship of the officer who co-ordinated the evacuation on the scene.
A boat carrying eight specially cast Jubilee bells will lead the water-borne procession, and churches along the river bank will return the peel as it passes.
Later on Sunday, Prime Minister David Cameron will attend a Jubilee party in Downing Street.
And around the country, a community initiative called the Big Jubilee Lunch will encourage people to share food with neighbours and friends in street parties or at picnics.
On Monday, a concert will be held in front of Buckingham Palace in the area surrounding the Queen Victoria Memorial, with performances from artists including Dame Shirley Bassey, Sir Elton John, Jessie J, JLS and Ed Sheeran.
When it ends at 22:00 BST, more than 4,000 beacons are due to be lit in the UK and around the world.
The Queen will light the UK's last beacon - the National Beacon - at about 22:30 BST, to be followed by a firework display at Buckingham Palace.