Written by By Judith Vonberg, CNN London

Queen Elizabeth II became the first British monarch to reign for 65 years on Monday.

Her Sapphire Jubilee has being marked with ceremonial gun salutes at London's Green Park and the Tower of London, and in several other locations across the United Kingdom. The bells at Westminster Abbey in central London were also rung in honor of the occasion.

A 41-gun salute is staged in Green Park, London to mark the Queen's Sapphire Jubilee. Credit: Jonathan Brady/PA Images/Getty Images

A photograph taken by British photographer David Bailey in 2014 has been reissued to celebrate the anniversary. In the photo, the Queen is wearing sapphire jewelry that was given to her as a wedding gift by her father King George VI in 1947.

This 2014 photograph has been reissued for Queen Elizabeth's Sapphire Anniversary on February 6. Credit: David Bailey via AP

The photo was originally taken for the GREAT Britain promotional campaign, which launched soon after the Queen's Diamond Jubilee celebrations, marking 60 years of her reign.

The Queen has no public events planned for the anniversary, and will instead spend the day at Sandringham, her country retreat in eastern England, according to a palace spokeswoman.

British Prime Minister Theresa May congratulated the Queen in a statement: "Today's Sapphire Jubilee marks yet another remarkable milestone for our remarkable Queen. I know the nation will join with me today in celebrating and giving thanks for the lifetime of service Her Majesty The Queen has given to our country and to the commonwealth."

The band of the Household Cavalry plays outside Buckingham Palace on February 6. Credit: Philip Toscano/PA Images/Getty Images

It was on February 6, 1952 that the Queen's father died and the 25-year-old princess ascended to the throne.

The Queen became Britain's longest reigning monarch in September 2015, after 63 years and 216 days on the throne, surpassing the record set by her great-great-grandmother, Queen Victoria.















1 / 8 Queen Elizabeth II's relationship with Land Rover extends back to 1948, when her father King George VI was presented with the 100th production vehicle. Pictured is Her Majesty in 1956 at the Badminton Horse Trials in Gloucestershire, England. Credit: Tidmarsh/Hulton Archive/Getty Images

Despite relinquishing some of her royal duties to other members of the family, she has continued to maintain an active public presence, appearing in a striking neon green outfit at her official 90th birthday celebrations in June 2016