The sprightly monarch, who still travels throughout Britain and the rest of the world, accepted flowers and gifts as she emerged from Windsor Castle, the centuries-old royal retreat west of London.
Dressed in a fuchsia pink overcoat and matching hat, Queen Elizabeth, smiling and even laughing at times, chatted with well-wishers of all ages who stood behind metal barriers.
The celebrations highlight how her popularity has endured despite the family divorces and scandals that shook the House of Windsor in the 1990s and prompted public speculation about how long the monarchy could survive.
Opinion polls suggest it will certainly last as long as she lives.
The Queen has received tens of thousands of birthday cards and e-mail messages from across Britain as well as from Australia, Canada and other former dominions and colonies in the 53-nation Commonwealth.
Prime Minister Tony Blair and his cabinet sent the Queen - Europe's longest serving monarch - a china tea set she wanted, estimated to be worth STG1,000 ($A2,418).
"I think she is so important that anything we can do to show her support is wonderful," said Maureen Vaughan, 73, who offered the queen white flowers for her birthday, one she herself shares with the sovereign.
The Union Jack flag flew from government buildings to mark the royal birthday, while a 21-gun salute - an annual tradition - was fired from Windsor Great Park.
Soldiers in London's Hyde Park also sounded a special 41-gun salute and cannons fired over Scotland's capital Edinburgh.
Queen Elizabeth herself enjoyed the first moments of her ninth decade in private before emerging from the castle, parts of which date back nearly 1,000 years, through the King Henry VIII gate.
The crowd gave her three cheers and joined in as the band of the Irish Guards, wearing their trademark red jackets and bear-skin hats, struck up "Happy Birthday To You".
The Queen, who wore a string of pearls, black gloves and a shiny black handbag, chatted with a small group of girl guides who curtseyed before giving her flowers and other small gifts.
Accompanied at first by her husband Prince Philip, 84, the queen then went on a 45-minute walkabout through Windsor town, chatting to some of the crowd estimated by police at 20,000-strong.
Supporters cheered and took souvenir pictures as she passed.
"Good gracious, thank you," the Queen said after being handed a big bunch of violas and daffodils by one well-wisher.
Later in the day, she travelled to Kew Palace in Richmond, west London, where her eldest son Prince Charles is hosting an 80th birthday black-tie dinner for her and 26 close family members.
On the menu was organic Scottish smoked salmon, roast loin of venison with a port wine sauce, served with steamed young cabbage and spring vegetables, and chocolate sponge cake.
Charles, heir to the throne, broadcast a warm tribute on TV, in which he said his mother had shown "remarkable steadfastness and fortitude" and remained a figure of "reassuring calm and dependability".
"For very nearly 60 of those 80 years she has been my darling Mama and my sentiments today are those of a proud and loving son who hopes that you will join with me in wishing the queen the happiest of happy birthdays," he said.
Born at 2:40 am on April 21, 1926, Elizabeth Alexandra Mary has grown to become one of the best-loved members of the House of
Windsor.
She was proclaimed Queen in February 1952, aged just 25, after the death of her father, King George VI, and was crowned in June 1953.
Married with four children - Charles, Anne, Andrew and Edward - the Queen juggled her job as head of state, the armed forces, the Commonwealth and the Church of England with motherhood and, later, being a grandmother.
The monarchy was rocked by the divorces of Charles, Anne and Andrew as well as by the death in a 1997 car crash of Princess Diana, who was overwhelmingly more popular than her ex-husband Charles.
An ITV television news poll published this week found that Queen Elizabeth is the most popular member of the royal family and more than half her British subjects want her to reign for the rest of her life.
Echoing an opinion given earlier this week by the Queen's cousin Margaret Rhodes, Countess Mountbatten, a close friend, confirmed to BBC radio that Queen Elizabeth sees her role as "a job for life" and will never abdicate.