The mother of Britain's Queen Elizabeth appeared before thousands outside Clarence House in a steely show of determination. The country's oldest royal had undergone an emergency blood transfusion for anemia on Wednesday.

She was on her feet for nearly 15 minutes as she took the salute of the Scots Guards marching with the band of the Grenadier Guards playing "Happy Birthday".

The Queen Mother also stood to receive bouquets and greetings cards and to greet a number of children.

She then greeted the crowds for three quarters of an hour from her chauffeur-driven golf cart, nicknamed the "Queen Mum Mobile".

"I'm feeling much better," she told people as they broke into a chorus of "Happy Birthday to you."

Her 100th birthday last year, when she became the royal family's first centenarian, was marked with lavish celebrations and an outpouring of affection from her admirers.

A crowd of 40,000 people surged towards the gates of Buckingham Palace to applaud Britain's best-loved royal.

In 70 years of service, the "Queen Mum" has remained a popular figure while other royals fell in and out of favour.

After her self-effacing husband unexpectedly took the throne in 1936, replacing his brother who had abdicated, she worked tirelessly to uphold the royal family's reputation and popularity as the old order and class barriers crumbled.

She became a key focus for morale during World War Two, defying German bombers by staying in London through the Blitz. Adolf Hitler described her as the most dangerous woman in Europe for her effect on public spirits.

A newspaper quoted unnamed royal sources earlier this week as saying the Queen Mother had told house guests at the Sandringham royal estate in eastern England that she wanted to become the country's oldest woman.

Amy Hulmes, aged 113, is believed to be Britain's oldest living woman.