From PM to ordinary people, UK commemorates Queen's 90th birthday

Though not a royalist, Jeremy Corbyn, leader of the opposition Labour Party, said the Queen has carried out her duties over the years "with extraordinary warmth."

Commoners, too, were celebrating -- and, some are meeting the monarch in person. The Queen is celebrating her 90th birthday at Windsor Castle, west of London.

She is unveiling a plaque marking the Queen's Walkway at the foot of Castle Hill, a long straight walkway that leads to the castle. And she and her husband, Prince Philip, who is 94, are on a "walkabout," greeting well-wishers.

Family members are paying tribute, too.

The Duke of Cambridge praised his grandmother for helping him cope with the loss of his mother, Diana, Princess of Wales.

"She's been a very strong female influence," Prince William said on Sky News. "And having lost my mother at a young age, it's been particularly important to me that I've had somebody like the Queen to look up to and who's been there and who has understood some of the more, um, complex issues when you lose a loved one."

Diana died in 1997 in a Paris automobile crash.

William also recalled an incident when his grandmother chastised him when he got into trouble with his cousins Peter and Zara Phillips while driving an all-terrain bike in Scotland.

He said: "We were chasing Zara around who was on a go-cart, and Peter and I managed to herd Zara into a lamppost. And the lamppost came down and nearly squashed her, and I remember my grandmother being the first person out at Balmoral running across the lawn in her kilt to tell them off."

She "came charging over and gave us the most almighty b****cking, and that sort of stuck in my mind from that moment on."

Last year, the Queen gained the title of Britain's longest-ever reigning monarch. On Thursday, she achieved another milestone as the nation's first sovereign to celebrate her 90th birthday.

Later in the summer, formal celebrations will be held to mark her "official birthday," culminating with a national service of thanksgiving at St. Paul's Cathedral and a giant picnic outside Buckingham Palace for 10,000 guests.

The monarch's celebrations began Wednesday with another first: an official photograph of four generations of royals, featuring three future kings posing at Buckingham Palace. The image of the Queen, and Princes Charles, William and George will be on special stamps issued to mark Elizabeth's life.

It's a milestone for 2-year-old Prince George, who makes his stamp debut, doubtless the first of many to come, pictured standing on a stack of blocks for a boost.