The West End suffers from petty crime and underage drinking

Unaccompanied youths spotted in Oxford Street, Piccadilly Circus and Trafalgar Square among other places, could be escorted home by police after 2100 BST.

Police say they will not stop everyone, but will concentrate on vulnerable lone children and gangs causing trouble.

Fifteen other areas, including Victoria and Paddington, will also be designated as "dispersal areas".

It follows a similar scheme in Somers Town, Camden, used to target gangs of youths and drug addicts, which came into force in June.

I don't think any responsible parent would like their 15-year-old unaccompanied in the West End after 9pm. Sir Ian Blair, Met deputy commissioner

When the prime minister and the home secretary visited the north London borough recently, they met residents who were extremely enthusiastic about the orders.

Police and councils have been given powers to disperse groups of two or more from an area if they are likely to cause "alarm, harassment or distress".

The West End is known for its cinemas, bars, video arcades and shops and many youngsters head there in the summer evenings.

But the area is also plagued by petty crime and police say it attracts gangs of young thugs and is a centre for under-age drinking.

Sir Ian Blair, the Met deputy commissioner, said the new powers, introduced under the Anti-Social Behaviour Act 2003, were "extremely appropriate".

"We don't want a blanket: 'This is miserable London' process," he said.

"We want to make it safe and enjoyable for as many people as possible."

He added: "I don't think any responsible parent would like their 15-year-old unaccompanied in the West End after 9pm."