Some of the crowd sang to the Queen as she stopped to chat

The band of the Irish Guards played Happy Birthday and the crowd cheered as she emerged from Windsor Castle.

The Queen - dressed in a vibrant pink coat and hat - accepted gifts, cards and flowers as she and Prince Philip walked around the town for 45 minutes.

Later she will join her family for a private dinner to be hosted by Prince Charles at Kew Palace.

Earlier, 300 well-wishers in New Zealand, the first Commonwealth country to celebrate the Queen's 80th, spelt out: "EIIR 80" on Government House's lawn, in Wellington.

The Queen earlier thanked the thousands of people who sent her cards and messages, saying they had helped make the day a "special one".

I have been very touched by what you have written and would like to express my gratitude The Queen

Buckingham Palace said the Queen had received 20,000 cards and 17,000 e-mails - sent via her 80th-birthday website.

She said: "I would like to thank the many thousands of people from this country and overseas who have sent me cards and messages on my 80th birthday.

"I have been very touched by what you have written and would like to express my gratitude to you all."

Royal walkabout

The Queen's birthday present from Prime Minister Tony Blair's Cabinet was a china tea set made by Spode pottery in Staffordshire, said Downing Street.

Mr Blair's official spokesman said Buckingham Palace had indicated it was "something the Queen specifically would like".

A spokesman for Spode, the UK's oldest pottery maker, said the set, from its Stafford Flowers range, would have cost about £1,000.

I just wanted to be involved and wish the Queen happy birthday Jennifer Hawkins

Also among her birthday messages was a special visual tribute from 500 crew members of a Royal Navy aircraft carrier, who lined up in formation to spell out 'Happy 80th'.

A photograph was taken from the air of the display on HMS Illustrious, which is in the Red Sea en route to a deployment in the Indian Ocean.

Well-wishers descended on Windsor in the early hours to greet the royal couple, some prepared for a long day with foldaway chairs and food provisions. Union Jacks and birthday banners were unfurled over police security barriers.

Thames Valley Police estimated the crowd exceeded 20,000.

Jennifer Hawkins, from Worthing, West Sussex, who was carrying an inflatable corgi, said: "I just wanted to be involved and wish the Queen happy birthday. She works so extremely hard and just presents herself so magnificently."

Mary Wintle, 71, originally from Gwaun Cae Gurwen, near Swansea, said: "I wanted to show we like the Queen in Wales. When she opened the Assembly, there were a few protesters but most people were supporting her."

Among the younger supporters was 17-year-old royalist Chris Foskett, who works in Tesco near Sandhurst.

DAY OF CELEBRATION 0830: Extra large ceremonial Royal Standard flag raised at Windsor Castle 1030: 21-gun salute on the Long Walk, Windsor Great Park 1200: 21-gun salutes in Cardiff and Fowey, Cornwall 1205: Queen and Prince Philip on walkabout in Windsor 1315 (approx): Giant 2000-signature birthday card from the City of London brought to Buckingham Palace 1700: BBC Radio 4 broadcast of tribute to the Queen by Prince Charles; on BBC One just before 6 O'Clock News 2040: Fireworks and laser show kicks off family dinner at Kew Palace, hosted by Prince Charles. About 2,000 people will watch the show

He said it was a "once in a lifetime" chance to be at Windsor for the big day.

"None of my friends would come with me but I think she is great-looking for her age, isn't she?

"She keeps the country together. You need somebody to represent your country, politicians can't do it."

Before heading off on the walkabout, which had to be extended to cater for the large crowds, the Queen received presents from youth organisations the Girls' Brigade England and Wales and the Scout Association, of which she is patron.

The celebrations for the monarch, who is the longest-serving in Europe, will be captured by television crews from around the world.

One of the first public displays to mark the day was the raising of a huge Royal Standard used for celebration days.

HAVE YOUR SAY [Give the Queen] a nice council flat and an old age pension - let her try to survive like normal pensioners have to James Whittaker, Preston, UK Send us your messages

The flag, which is 38ft long and 19ft wide (11.5m x 5.8m), was raised over the Round Tower, the highest point of Windsor Castle, at 0830 BST.

It was followed at 1030 BST by a 21-gun salute in Windsor Great Park.

Prince Charles will deliver a birthday tribute to his mother later, which will be broadcast on BBC Radio 4 at 1700 BST.

And his message is to be screened on BBC1 and Sky News at about 1755 BST and on ITV1 during the ITV evening news.

The climax of the birthday festivities will see the Queen and members of the Royal Family gather at Kew Palace in the Royal Botanic Gardens Kew for dinner.

On this special 80th birthday of my mama I am most grateful to you for your kind wishes, which I shall certainly pass on this evening at dinner Prince Charles

Charles earlier thanked well-wishers at the Gordon Highlanders Museum, Aberdeen, which, as patron, he was officially reopening after a £1.2m refurbishment.

"On this special 80th birthday of my mama I am most grateful to you for your kind wishes, which I shall certainly pass on this evening at dinner."

Speaking on BBC Radio 4's Today programme on Friday the Queen's close friend Countess Mountbatten said she probably found her public engagements tiring as she approached 80.

She said: "I'm 82 myself and I know one gets tired more quickly, she has great stamina and extraordinary energy and she probably doesn't feel it quite as easy as when she was a young woman but she just keeps going, bless her."

She also said she hoped everyone in the country realised what a "precious thing" it was to have a monarchy.