LONDON (REUTERS).- They have been gathering dust in a basement for more than 40 years, but now U.S. photographer Mike Mitchell has decided to auction a group of pictures which capture the moment the Beatles became a worldwide phenomenon.

Mitchell, now in his mid-60s, was given a press pass to the Fab Four's first U.S. concert at the Washington Coliseum in 1964, just two days after their breakthrough television appearance on The Ed Sullivan Show.

He was back later that year to cover their concert in Baltimore, by which time their fame had grown considerably.

"I heard the music and I had to be there," said Mitchell, surrounded by a selection of the black-and-white images which had a spontaneity that many later photographs lacked.

He is selling the collection through Christie's auctioneers in New York on July 20, and is exhibiting them in London first to raise awareness among potential bidders.

"Things were much different back then," he told Reuters on Friday. "There was no big security presence, the press wasn't corralled and I was free to sort of embrace my own ambition."

Several of the pictures, valued at between around $1,000 to $6,000 each, are taken from unusual vantage points and focus on particular details.

The photograph chosen by Christie's to illustrate the collection shows the four Beatles from behind looking into the bright lights that would follow them wherever they went after the outbreak of "Beatlemania" in 1964.

Another was taken from the side of a table behind which the four musicians sat for a press conference, while others focus solely on Ringo Starr's hands or Paul McCartney's feet on a stage littered with sweets thrown by screaming fans.

"SQUEEZED" BY HOUSING CRISIS

Mitchell, who was 18 when he took the photographs, said that by the 1970s he knew he had been privileged to be a part of rock and roll history.

And when he was caught out by the recent U.S. housing crisis he decided it was time to dust off his archive and sell them. The collection is estimated to be worth around $100,000.

"You cannot forget 8,000 screaming girls," he said of his earliest memories of the Beatles. "It was like the birth of my generation."

He said that he tried to do what other photographers at the events were not doing, explaining the personal style of images.

Cathy Elkies, head of Iconic Sales at Christie's in the United States, said she knew she was on to "something extraordinary" the moment she saw the images via email.

"There's a lot of Beatles images out there, no question," she said. "(But) they are amazing, highly intimate, high-access kinds of images ... To find a treasure trove of art -- you just don't find that any more."

She said the surviving two Beatles and the families of John Lennon and George Harrison may well come to see the pictures before they go on sale.

Elkies added that the relatively modest prices meant buyers could range from serious pop collectors to private individuals wanting a piece of Beatles history.

(Reporting by Mike Collett-White, editing by Paul Casciato)

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