CONGRATULATIONS poured in from around the world yesterday as the Queen Mother celebrated her centenary and one simple card summed up the joy of the British nation.

It was from her daughter and was signed ''Lilibet'' - her childhood name - in place of the formal Elizabeth R.

It was a tumultuous day of celebration for the Queen Mother and the nation, but even at 100 years old, she was last night out on the town continuing the festivities.

The royal family's first centenarian was at the Royal Opera House in London's Covent Garden to see a ballet with her daughters, the Queen, 74, and Princess Margaret, 69.

Earlier, surrounded by 27 royals and 40,000 other well-wishers, she took the nation's salute on the balcony of Buckingham Palace.

Britain's best-loved royal smiled and, with a familiar wave, acknowledged the crowds who sang Happy Birthday, cheered and waved Union Jacks. Some had camped out for two nights to get a good view.

Thousands of Scots demonstrated their fascination with Britain's best known grandmother as they turned out for her birthday celebrations across the country.

Glamis Castle, in Angus, where she spent much of her childhood, became the focus for a spectacular birthday party.

Stirling Castle was also a focal point for celebrations, with four Tornado jets from 43 Squadron flying from RAF Leuchars in Fife, and passing over the castle in diamond formation.

In Glasgow, Lord Provost Alex Mosson sent floral displays and cakes to the residents of the city council's residential homes for the elderly.

In the capital, tourists and locals lined the cobbled paths and ramparts of Edinburgh Castle and watched as the 105 Regiment Royal Artillery (Volunteers) fired 21 shots to mark the occasion.

Prime Minister Tony Blair paid tribute to the Queen Mother's ''extraordinary sense of duty'', and the Queen issued a special Honours List marking her mother's 100th birthday and recognising the work of nine of her close supporters.

The Queen Mother was said to be ''very touched'' by the warm reception and ''deeply appreciated'' the many tributes from Britain and around the world.

At 11.45am, dressed in her favourite shade of light blue, she walked slowly, with two sticks, to the main gates of Clarence House. Close by was her eldest grandson, the Prince of Wales, sporting a blue cornflower in the lapel of his double-breasted grey suit.

Then came the first public tribute of the day as the King's Troop Royal Horse Artillery rode past, followed by the band of the Irish Guards, playing Happy Birthday, with Grenadier Guards marching behind.

Standing in the centre of the open gates, the Queen Mother smiled broadly and waved.

The Queen's postman, 58-year-old Tony Nicholls, then arrived in a bright red, electric-powered Royal Mail van with a special delivery, handing the Queen Mother her 100th birthday card from the Queen.

''Thank you very much,'' said the Queen Mother. ''If I can get it open.''

She turned to her equerry, Captain de Rouet, and asked: ''Can you use your sword?''

To the amusement of the Prince of Wales, the Irish Guards officer drew his sabre and slit open the white envelope.

Inside was the 100th birthday card, which has now replaced telegrams and telemessages.

The personalised handwritten message inside the Queen Mother's royal birthday card read: ''On your 100th Birthday all the family join with me in sending you

our loving best wishes for this special day. Lilibet.

Mr Nicholls, from south-west London, has worked for the Royal Mail for 34 years and has been based at Buckingham Palace in the Court Post Office for nine years.

His normal job is to deliver mail addressed to the Queen wherever she is - at the Palace, at Windsor Castle, at Sandringham in Norfolk, or at Balmoral.

The card, in an envelope with the Buckingham Palace crest, was similar to cards delivered to 11 other centenarians yesterday.

Next it was the turn of junior housemaid Nicola Lewis to present flowers to the Queen Mother from staff at Clarence House.

Ms Lewis, 21, from South Wales, who has worked for the Queen Mother for two years, had been practising her curtsy and wished the Queen Mother a very happy birthday.

The Queen Mother stood throughout and declined to sit in a chair brought by her long-serving steward, Billy Tallon.

But the well-wishers had to wait a few minutes before the birthday royal appeared in her open carriage bound for the palace.

At noon, the royal carriage, pulled by greys from the Royal Mews set out on its route to the palace along Stable Yard Road, Cleveland Row, Marlborough Road, and The Mall and was escorted by 19 cavalrymen.

Police estimated 40,000 people packed The Mall from Clarence House, the Queen Mother's London residence, to Buckingham Palace, to see the her ride in an open carriage with Prince Charles.

''We want the Queen Mum,'' the large crowd in The Mall chanted - and the Queen Mother waved.

A 41-gun salute was fired from nearby Green Park as they entered the palace forecourt.

As she stepped out on to the balcony, the band of the Coldstream Guards, in the palace forecourt below, played Happy Birthday To You and one of the Queen Mother's favourite pop songs, Congratulations, and she moved her hands and arms to the rhythm of the music as the band struck up the Cliff Richard hit.

She took the nation's salute - every royal wave met by a loud cheer - with the Queen and Princess Margaret, who were soon joined by the rest of the family.

Prince William, 18, towering over the group at 6ft 2ins, stood chatting to his father, with his younger brother, Prince Harry, 15, nearby.

All senior royals celebrated with the Queen Mother at a private family lunch in the palace's Bow Room.