The unidentified 40-year-old man climbed the gates to the Queen's London home in front of a crowd of tourists on Wednesday afternoon.

He crossed the forecourt towards the Privy Purse Door - an entrance manned by footmen and police during the day - before being wrestled to the ground by police officers, handcuffed and led away.

The BBC's Nicholas Witchell: "There is more protection than is sometimes apparent"

He was detained under the Mental Health Act and was taken to a central London psychiatric hospital for evaluation.

A Scotland Yard spokesman said the man would not be prosecuted.

The Queen was in the palace at the time of the incident, which occurred just a few minutes before the arrival of her second son, the Duke of York.

The arrest was captured on film by Australian tourist Dave Killian, a 33-year-old builder from Adelaide.

He told the Evening Standard newspaper: "He must have run at least 60 metres across the courtyard, sprinting like the clappers and chased the whole way by the police officer.

"Then about two minutes later, Prince Andrew turned up in his limousine. We've just come here for a day's sight-seeing. It's not what you expect."

String of scares

BBC Royal Correspondent Nicholas Witchell says the incident proves that the Royal Family is very securely guarded, despite the low profile of police at the palace.

And royal protection officers are given special training in dealing with those with mental health problems, he said.

The incident is the latest in a string of security scares at royal palaces.

The most famous incident came in 1982 after Michael Fagan, who suffered from mental problems, twice got into the palace itself.

On the second occasion, he climbed a drainpipe into the Queen's bedroom and spent 10 minutes sitting on her bed holding a broken ashtray and talking to her before guards arrived.

In 1993, anti-nuclear demonstrators scaled the palace walls with ladders and sang on the lawn before being arrested.

In 1994, a paraglider landed on the palace roof as a stunt, and in 1995, a man rammed the palace gates in a car.