Britain's Queen Elizabeth smiles as she departs after officially opening the Scottish Borders Railway at Tweedbank Station in Scotland, Britain September 9, 2015.[Photo/Agencies]

EDINBURGH - Queen Elizabeth sealed a special place in the Britain's history by becoming its longest-reigning monarch on Wednesday, but amid warm tributes from politicians and the public, she said the landmark was not something to which she had ever aspired.

Elizabeth, 89, surpasses the 63 years, 7 months, 2 days, 16 hours and 23 minutes that her great-great-grandmother Queen Victoria spent on the throne.

The occasion was marked by cheering, flag-waving crowds on the street, bells ringing out in Westminster Abbey and solemn messages in parliament but Elizabeth, who is also the nation's oldest ever monarch, wanted little fuss.

She made only a brief reference to it in a speech as she opened a new railway line in Scotland.

Thanking the crowd for their welcome, she said: "Many ... have also kindly noted another significance attaching to today, although it is not one to which I have ever aspired.

"Inevitably a long life can pass by many milestones - my own is no exception - but I thank you all and the many others at home and overseas for your touching messages and great kindness."

However, in London, political leaders heaped praise on a head of state who became monarch aged just 25 at a time when Britain was emerging from the ravages of World War II and has witnessed massive political change, social upheaval and the end of the British empire during her long reign.

"The Queen is our Queen and we could not be more proud of her," Prime Minister David Cameron told parliament in London calling her a "rock of stability". "She has served this country with unfailing grace, dignity and decency and long may she continue to do so."

In central London, the royal barge Gloriana led a flotilla of boats down the River Thames and past a four-gun salute from the battleship HMS Belfast, now permanently moored on the river.