AFP/London

Britain celebrated Queen Elizabeth II becoming the country's longest-reigning monarch on Wednesday, a milestone the 89-year-old herself downplayed as like many in the life of an elderly person.

The bells of Westminster Abbey rang out in London as Prime Minister David Cameron paid tribute in parliament to her 63-year reign as a "golden thread running through three post-war generations".

As she opened a railway line in Scotland after riding along it on a steam train, the queen said the landmark was "not one to which I have ever aspired".

"Inevitably, a long life can pass by many milestones. My own is no exception," she told a cheering crowd of supporters waving British and Scottish flags.

"But I thank you all and the many others at home and overseas for your touching messages," said the queen, who wore a diamond-studded brooch that once belonged to the previous record holder -- her great-great-grandmother Queen Victoria.

It is not known where exactly she will be at 1630 GMT, the best estimate from royal officials for the time at which the monarch, who has become synonymous with Britain itself, reaches the historic moment.

At that time, she will have served 23,226 days, 16 hours and roughly 30 minutes on the throne -- a reign spanning from the gloom of the post-war years to the digital age, with a few royal scandals causing blushes along the way.

She will reportedly host a private dinner later on Wednesday at Balmoral Castle -- her traditional summer residence in Scotland -- with her grandson Prince William and his wife Kate in attendance.

"She is our queen and we could not be more proud of her," Cameron told parliament. "She has served this country with unerring grace, dignity and decency and long may she continue to do so."

- 'Extraordinary achievement' -

Elizabeth became queen upon the death of her father George VI, Britain's king during World War II, whose youthful stutter inspired the Oscar-winning film "The King's Speech". Royal sources said she was not keen to celebrate because of the painful memory of his death.

The calculation of her time on the throne is based on when George VI passed away, which is estimated at around 1:00 am on February 6, 1952 -- an hour after he was seen for the last time at his bedroom window at Sandringham House in eastern England.

The queen presided over a gradual decline in Britain's global influence as many of its former colonies became independent, as well as a marked rise in living standards and closer integration with Europe.

Her reign has lasted through 12 British prime ministers, starting with wartime leader Winston Churchill, who was a mentor to the young queen.

She has steered the monarchy through some rocky patches, including the collapse of three of her children's marriages and public anger at her reaction to Princess Diana's death in 1997, which some saw as cold.

The royal family has since tried to present itself as more in touch with the public.

That decision was crowned by the highly-popular marriage of William and commoner Kate Middleton in 2011, and the birth of the couple's two children, Prince George and Princess Charlotte.

Outside Buckingham Palace on Wednesday, wellwishers from around the world gathered to celebrate.

"It's a very special day, we are very privileged to be here, to share this moment with everyone else in the world," said Janice Gallagher, 68, from Australia.

"She provides good leadership, she's very calm and composed".

Town crier Tony Appleton read the crowd a message before leading them in a chorus of cheers for the queen.

As part of the celebrations, the Royal Mail announced that first-class stamps would be regal purple for the next year while London's iconic BT Tower beamed the message "Long May She Reign" across the capital.

Historian Jane Ridley said it was "remarkable" that the queen had been able to maintain the monarchy's popularity, despite a global trend towards republicanism.

Ridley attributed her success to the fact that, while she is constantly meeting the public, the queen keeps her thoughts very much to herself.

"She has done it very cleverly. She has never given an interview. She is constantly seen."

But Graham Smith, head of the campaign group Republic, said the occasion was nothing to celebrate.

"The queen has survived this long in her role as head of state for one reason, she has never had to face election or be held to account," he said.