Thousands of people are lining 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 fleet.
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 decorated with almost 10,000 cut flowers.
Tomorrow a concert will be held in front of Buckingham Palace, ending with a firework display and more than 4,000 beacons are due to be lit in the UK and around the world.