Britain celebrates Queen Elizabeth II's 90th birthday on Thursday, April 21 with her eldest son Prince Charles paying tribute in a special radio broadcast and Prime Minister David Cameron leading a parliamentary homage, AFP says.

The sovereign, who last year overtook her great-great grandmother queen Victoria to become Britain's longest-reigning monarch, is to mark the day in Windsor Castle with official engagements and a dinner with the royal family.

The excitement was visible in the small town of Windsor, west of London, with special bunting put up in the streets and local shops selling souvenir tea and cakes as well-wishers gathered to celebrate an historic day.

As military gun salutes are fired in London, Queen Elizabeth will unveil a plaque in Windsor for a new local walking tour in her honor and set fire to the first of hundreds of beacons being lit up around the kingdom.

Buckingham Palace released three official pictures of the monarch taken by U.S. celebrity photographer Annie Leibovitz. They were shot in Windsor Castle, where she will host U.S. President Barack Obama and First Lady Michelle Obama on Friday.

The royal family's popularity plunged after the death of Diana, Charles' first wife, in a car crash in 1997, which came soon after three of the Queen's four children got divorced and Windsor Castle was partially gutted by fire.

But overall Queen Elizabeth has shown an ability to unite her subjects through tumultuous times -- from the grim post-World War II years through the decline of the British empire to troubles that have rocked the monarchy, AFP says.

Support for the monarchy remains high at 76 percent, according to an Ipsos-Mori poll for King's College London.