Gun salutes marked the 64th anniversary of the Queen's accession to the throne.
Cannons were fired in public for the Queen, now the longest reigning monarch in British history.
It commemorates the start of her reign, which began on the death of her father, King George VI.
The Queen is likely to be reflecting on the anniversary of her father's death in private at Sandringham, where she has been staying for her annual winter break.
George VI died in his sleep at Sandringham House on the royal estate in Norfolk on February 6 1952 after suffering from lung cancer.
Princess Elizabeth, who was just 25, was thousands of miles away in Kenya on a Commonwealth tour with the Duke of Edinburgh when she learned of his death. She returned home a queen.
A 41-gun salute was fired by the King's Troop Royal Horse Artillery in London's Green Park and a 62-gun salute by the Honourable Artillery Company at the Tower of London.
Soldiers from The King's Troop, the ceremonial saluting battery of the Household Division, also took up positions in Hyde Park.
The Band of the Royal Artillery played a selection of celebratory music close to the firing position.
Seventy-one horses pulled six First World War-era 13-pounder field guns into position in the park where a 41-gun Royal Salute was fired.
A 62-round Royal Salute from Gun Wharf - with an extra 21 volleys for the citizens of the City of London to show their loyalty to the monarch was fired at the Tower of London.
Later in the year the Queen has a milestone birthday in April when she will turn 90, and a weekend of national events are planned to coincide with her official birthday celebrations in June.