Queen acceded the throne on February 6, 1952, after the demise of her father King George VI from lung cancer.
The Queen was thousands of miles away from Buckingham Palace in Kenya when she learnt of her accession to the throne.
She was spending the night at a hotel when her husband Prince Philip broke the news that King George VI had died.
The couple returned to Britain and she was swiftly proclaimed Queen. Her coronation came more than a year later on June 2 1953.
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The King's Troop Royal Horse Artillery were the first to mark the occasion in the capital with a 41-gun salute in Hyde Park at midday.
It was followed by a 62-gun salute by the Honourable Artillery Company from Gun Wharf at the Tower of London at 1pm.
A 21-gun salute was also held by the 4th Regiment Royal Artillery in York's Museum Gardens.
The 87-year-old Queen is preparing herself for a busy period of appointments over the next few months.
In April the Queen and the Duke of Edinburgh will also visit Rome to meet Pope Francis at the Vatican.
The Queen is the second-longest-reigning monarch in the UK behind Queen Victoria who served for 63 years and 216 days.