Hundreds of thousands of people have lined the river to watch the Queen’s barge in 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.
The Queen and other senior royals are aboard a lavishly-adapted royal barge called the Spirit of Chartwell.
Decorated with almost 10,000 cut flowers, it will travel from Albert Bridge to Tower Bridge.
The Duke and Duchess of Cambridge and Prince Harry boarded the launch at 14:10 BST, followed by the Queen who was greeted by a spontaneous rendition of the national anthem from the crowds.
Earlier, Prince Charles joined a London street party in Piccadilly and concerts and events are taking place UK-wide.
The spectacle, which began with mustering at 11:30 BST, started with the ringing of Jubilee bells at 14:40 BST and will end at about 17:30 after the last vessel completes the seven-mile route at Tower Bridge.
The Thames barrier was 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.
Crowds more than 10 deep are lining the banks of the Thames at Battersea Park.
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.
“They will set the pace and behind them will be the Queen… with what is called the VIP squadron, and behind that historic boats and passenger boats,” said Richard Everitt, chief executive of the Port of London Authority.
Mr Everitt said a “safety hard shoulder” was being set up on the north side of the Thames so any boats that broke down or experienced problems could be assisted.
Vessels have to stick to a pre-arranged formation, keeping a boat’s length apart from those in front of and behind them, because “if one boat starts to veer, they all will”, said pageant co-ordinator Ian Welsh.
Adam Kerr, the skipper of a restored 19th Century Cornish fishing lugger, the Barnabas, sailed 400 nautical miles to participate, and predicted a “pretty colourful spectacle”.
“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,” he said.
Prime Minister David Cameron paid tribute to the 86-year-old Queen during an interview on BBC One’s Andrew Marr programme, saying: “Her insight and her sharpness is extraordinary and I don’t see any sign of her working less hard.” But his own Jubilee party in Downing Street has been moved indoors because of the weather.
In Piccadilly, central London, the BBC’s Sangita Myska said there was a “huge crush” as well-wishers flocked to greet the Duke and Duchess of Cornwall at the street party where 500 tables have been set up.
In other news:
* There are reports of people heading to Jubilee events having difficulties getting to London because of overcrowded trains. A BBC reporter says scores of people have been left on platforms on stations en route from Birmingham, with 150 unable to board a train at Banbury, Oxfordshire. Elsewhere, services out of Peterborough were delayed by over-running engineering works
* BBC Big Screens are transmitting live coverage of the pageant in 22 locations around the UK including Belfast, Cardiff, Edinburgh, Birmingham, Bristol, Leeds, Liverpool, Manchester and Middlesbrough
* The Big Jubilee Lunch initiative is encouraging people to share food with neighbours and friends in street parties and picnics
* Thousands will watch the flotilla from Battersea Park, at a festival celebrating music, food and fashion from the past 60 years
* Morecambe, Lancashire, is aiming for the longest street party by beating the 1.5 miles (2.4 km) event held in Combe Martin, Devon, for the Queen’s Golden Jubilee in 2002
* Almost 9,500 road closure applications have been made across England and Wales, with Hertfordshire topping the street party list with 451 event licences
* Glenn Allison, the commanding officer of the 727 Naval Air Squadron, says he will realise a life’s ambition when he takes to the skies in the cockpit of a Swordfish bi-plane for the flypast to mark the pageant’s finale
* A third of all of Scotland’s street party applications have been made in Edinburgh, where 10 official events are scheduled
* Hundreds of street parties are taking place in Wales but the weather has forced some to be postponed. The beach party at Rhosneigr close to the Anglesey home of the Duke and Duchess of Cambridge was rained off, but celebrations moved indoors.
* Jubilee events are being held across the Commonwealth. In the Arctic, explorers from Devon are holding a party. In New York, 10 UK-registered yachts will fly union jack spinnaker sails in a salute. In Afghanistan, troops at Camp Bastion held parties complete with bunting
The £10m cost of the event has been met by private donations but the security costs will be paid for by the taxpayer.
Anti-monarchy group Republic is holding a demonstration against what it calls an unelected, unaccountable monarch.
Pageant watchers were advised to wrap up with waterproofs and wind proofs amid temperatures of around 11C.
BBC Weather presenter Sara Thornton said there would be dry interludes but it was “going to be cloudy and will feel cold and damp” in London.
Party-goers in most of England and Wales should expect cloud and the threat of rain, while Scotland and Northern Ireland would be dry with some sunshine.
Rainy weather was not enough to deter hundreds of people who camped out overnight along the Thames to claim the best spots to view the flotilla.
A collection of small ships used to rescue stranded troops from the beaches of Dunkirk in 1940 will also take part, led by the Motor Torpedo boat 102, the flagship of the officer who co-ordinated the evacuation.
A boat carrying eight specially cast Jubilee bells will lead the water-borne procession, and churches along the river bank will return the peal as it passes. There will be 10 musical barges, carrying choirs and orchestras.
On Monday, 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.