Queen Elizabeth II
Queen Elizabeth II arrives at the Queen Elizabeth II delivery office in Windsor with Prince Philip, Duke of Edinburgh on April 20, 2016 in Windsor, Britain, a day before her 90th birthday. Reuters/Chris Jackson

Queen Elizabeth II, the longest ever serving monarch of Britain, has turned 90. Huge crowds lined outside Windsor Castle to wish the Queen as she greeted well-wishers.

The Queen even sent her best wishes to those celebrating their 90th birthday. "I send my best wishes to those who are celebrating their 90th birthday...on this shared occasion, I send my warm congratulations to you," the 90-year-old monarch said.

In a heartfelt tribute to Her Majesty, the royal palace has released a series of pictures to honour the Queen's family life and official role. One of the pictures show her surrounded by her two youngest grandchildren and five great-grandchildren, including Prince William and Kate Middleton's children, Prince George and Princess Charlotte.

The Royal Mail has issued six special stamps touching upon the Queen's life to commemorate Her Majesty's 90th birthday. A specially commissioned stamp sheet of four generations of the royal family, including the Queen and future heirs to the throne that includes her eldest son Prince Charles (Prince of Wales), her eldest grandson Prince William (Duke of Cambridge) and William's son Prince George has also been issued.

Queen Elizabeth II
Britain's Queen Elizabeth II poses with her five great-grandchildren and her two youngest grandchildren in the Green Drawing Room, part of Windsor Castle's semi-State apartments. Reuters Handout

The Queen was born at 2.40am on 21 April 1926. Unlike the current tradition in which a royal birth is announced in written on an easel outside Buckingham Palace, Elizabeth's birth was officially announced in a court circular. She was proclaimed the Queen of England during coronation in 1953 at the age of 27 upon the death of her father, King George VI.

Queen at 90: Royals pay tribute

The royals are paying a special family tribute to the Queen on the historic occasion in a television programme led by the Queen's son and the heir to the throne, the Prince of Wales. The programme will be broadcast BBCOne.

In a documentary by ITV shot last month to honour the Queen on her 90th birthday, the royal family members revealed about their close relationship with Her Majesty. Kate Middleton said that the Queen is very caring and has been a constant support to her.

"She's [the Queen] been very generous in not being forceful at all and in any of her views, but I feel she's been there, a gentle guidance really for me," the Duchess of Cambridge said.

Prince William, 33, seconded saying that his granny always gave him a sense of protection. "Growing up, having this figurehead, having this stability above me has been incredible. I have been able to explore, understand, slightly carve my own path. I greatly appreciate and value that protection."

Prince Charles said: "When you think that all these years the Queen has been on throne, that in itself is a huge achievement, coping with so many different challenges and complications and always being there really in that remarkable way – steadfast."