LONDON (AFP) - Cradled in her mother's arms, Britain's new sleeping princess was welcomed into the outside world Saturday with joyous cheers, a forest of camera phones and a sea of flags.

It was both a traditional and very 21st-century moment as Prince William and his wife Kate emerged from the Lindo Wing doors at St Mary's Hospital in London.

Several hundred cheering well-wishers crammed behind barricades on the streets around the private hospital wing, screaming with delight as the beaming couple emerged.

Wearing a lemon and white floral summery dress, a smiling Kate cradled her daughter in her right arm, the baby wrapped in a white blanket and wearing a white bonnet.

The princess, less than 12 hours old, was sound asleep despite the rousing cheers and camera lens clicks from more than 100 news photographers.

William, wearing a white shirt, blue pullover and navy trousers, held his wife's hand as he gently helped her down the hospital steps.

"It was an absolutely fantastic moment. I'm touched by it," said royal "superfan" John Loughrey, who had camped out for 12 days to get a glimpse of the new princess.

"It is a treasure I shall remember forever," he told AFP, dressed top to toe in Union Flag attire.

"I got tireder and tireder. It was hard! But at the end of it, it's worth it," he said.

'A MOMENT IN HISTORY'

The new baby is fourth in line to the throne of 16 realms, including Britain, Canada, Australia and New Zealand.

Sarah Schlederer, 28, from Sydney, was there with her family who had just arrived from Australia on holiday.

"All the hands went up with the cheers and the cameras so I saw it through my iPhone," she said.

"It was exciting. There was screaming and cheering because people have been waiting so long and it's a moment in history that's never going to come again, so to be here and be a part of it is great."

Wily Sheila Riordan, 86, bagged herself an excellent window-side view from a hospital waiting area opposite the Lindo Wing doors, defying 11 hours of hospital staff clearing out other loitering royal-watchers.

"It was worth the wait, every bit of it," she said.

"We're royalists. It means everything to my family, going way back generations of Londoners. William looked handsome," she added.

William and Kate went into hospital at 6am and the baby was born at 8.34am.

The news of the royal family's latest arrival was hollered out by a traditional town crier with a bell on the hospital steps.

GEORGE MEETS HIS LITTLE SISTER

William emerged mid-afternoon to drive home to nearby Kensington Palace and fetch his son Prince George, giving the one-year-old a first glimpse of his new baby sister.

Himself born in the Lindo Wing in July 2013, George has rarely been seen since in Britain - another rare treat for royal watchers.

William put George down on his feet but the toddler wanted to be picked up again, so William carried him along, saying "wave to everybody", which the young prince duly did.

William planted a kiss on his son's temple as the two future kings headed inside, George looking back over his father's shoulder at the tightly-packed bank of news photographers.

George later left through a back door but William and Kate emerged with their new princess at 6.11pm.

"I saw William and Kate and the baby on the steps!" said an enthused Doreen Hyde, who was wearing her beloved "jubilee raincoat" of miniature British flags.

"It was a real party atmosphere. We had a bit of champagne and sweets and cake," said the 69-year-old from Liverpool, north-west England.

"It was electric, everyone was full of excitement, trying to get pictures of the baby."

The Duke and Duchess of Cambridge went back inside for five minutes before William emerged with his daughter in a car seat, putting her safely in the back alongside her mum.

With one last wave, William drove them home himself to start their new life as a family of four.