A beaming Queen Elizabeth waved to cheering crowds massed outside Buckingham Palace on Thursday as Britain kicked off four days of pomp, parties and parades to celebrate her record-breaking 70 years on the British throne.

Tens of thousands of supporters waving flags lined the streets of London for a military parade at the start of the four-day Platinum Jubilee. Millions of people across Britain and the world were expected to watch the festivities, join street parties and light beacons in honour of the 96-year-old monarch.

Elizabeth, holding a walking stick and wearing a dusky dove blue outfit that she also wore for an official Jubilee photograph, was joined by her son and heir Prince Charles, 73, and other senior royals on the balcony of Buckingham Palace.

Britain's current head of state has been on the throne for longer than any of her predecessors, and is the third-longest reigning monarch ever of a sovereign state. Opinion polls show she remains hugely popular and respected among British people.

The celebrations began with the Trooping the Colour, a military parade held annually to mark the Queen's official birthday, where 1,500 soldiers marched to military music in ceremonial uniforms of scarlet tunics and bearskin hats.

Later the crowds moved to the Mall, the grand boulevard running up to Buckingham Palace, where in brilliant sunshine they cheered and waved Union flags as a display of modern and historic planes took place overhead. Fifteen Typhoon jets spelled out the number 70.

World leaders including US President Joe Biden, France's Emmanuel Macron, Pope Francis and former British prime ministers were among those sending messages of goodwill.

"Thank you to everyone who has been involved in convening communities, families, neighbours and friends to mark my Platinum Jubilee, in the United Kingdom and across the Commonwealth," the Queen said in a statement as the festivities got under way.

"I continue to be inspired by the goodwill shown to me." (Reuters)