Video report by Tim Ewart, ITV News Royal Editor
The Queen has thanked the nation for its "touching messages of great kindness" as she becomes Britain's longest-reigning monarch - though Her Majesty said she "never aspired" to reach the milestone.
Prime Minister David Cameron led tributes to the record royal, describing her time on the throne as a "golden thread running through three post-war generations".
The Queen halted her holiday in Balmoral to mark the day by opening the new £294 million Scottish Borders Railway and made a small reference to the historical significance of the occasion in a brief speech.
"A long life can pass by many milestones. My own is no exception," Her Majesty said, adding that she was "thankful" for the "touching messages" she had received from around the world.
Her Majesty replaces her great-great-grandmother Queen Victoria in the record books after spending more than 23,226 days as monarch.
The latest day of service saw the Queen lead a royal party on the steam locomotive, the Union of South Africa, alongside Scotland's First Minister Nicola Sturgeon.
The luxury train transported the royal party from Waverley Station in Edinburgh to the Borders town of Tweedbank, with a brief stop-off to meet well-wishers at Newtongrange.
The Queen had been keen to play down the occasion after inheriting the throne far earlier than expected upon the death of her father George VI in 1952.
Speaking in the House of Commons, the Prime Minister joked that he risked disobeying the monarch by marking the historic day with a dedicated speech.
But Mr Cameron said: "I do think it's right that today we should stop and take a moment as a nation to stop and mark this historic milestone."
Acting Labour leader Harriet Harman said the Queen is "admired by billions of people all around the world" in a following tribute in the Commons.
Ms Harman praised Her Majesty's "tireless" commitment and dedication to the service of her country and joked that the monarch had "gone from sending telegrams to sending tweets" during her record reign.
The Duchess of Cornwall added her own tribute to her mother-in-law as she paid a visit to ITV's TV studios in London.
Events were held across the country to mark the day, with a nine-boat flotilla meanwhile travelling down the River Thames in central London and HMS Belfast sounding a special four-gun salute.