Thousands of people are lining (3rd June 2012) the Thames in London for the Jubilee river pageant despite wet weather, as street parties get under way nationwide.
Hundreds of thousands of people are expected to watch the Queen's barge lead a 1,000-strong flotilla.
The nautical parade, marking 60 years of her reign, promises to be the most spectacular in London for 350 years.
Prince Charles has joined a London street party in Piccadilly and concerts and events are taking place UK-wide.
Meanwhile, some people heading into London for the celebrations have been delayed because they have been unable to get on crowded trains.
When the pageant begins, 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 10,000 cut flowers.
The spectacle, which began with mustering at 11:30 BST, will end at about17:30 after the last vessel completes the seven-mile route at Tower Bridge.
The Thames barrier is being lowered to slow the river's flow. Some 20,000 people are expected to be in the boats of the flotilla, which will travel at 4 knots (4.6 miles) an hour.
Boats heading to the muster points at Hammersmith and Battersea bridges sounded their horns and were met with cheers from crowds on the banks as bells from churches sounded. A steady stream of rowing and paddle boats headed up to the front of the flotilla.