LONDON (AP) — British Prime Minister David Cameron has called Queen Elizabeth II "a rock for our nation" as the monarch celebrates her 90th birthday.

The queen is spending the day at Windsor Castle, near London, with her family.

Cameron said Thursday that Britain's longest-reigning monarch "has lived through some extraordinary times," from World War II to the moon landing, the end of the Cold War and advent of peace in Northern Ireland.

He said that "Her Majesty has been steadfast — a rock of strength for our nation, for our Commonwealth and on many occasions for the whole world."

Buckingham Palace issued three portraits by photographer Annie Leibovitz to mark the day. One shows the queen surrounded by seven young grandchildren and great-grandchildren, including 11-month-old Princess Charlotte, whom she holds in her arms.

Another shows the queen walking in the grounds of Windsor Castle with four of her beloved dogs — corgis Willow and Holly and dorgis Vulcan and Candy.