Smash-and-grab raiders targeted jewellers at Europe's largest urban shopping centre in the early hours of today.
Valuables worth tens of thousands of pounds were stolen when a gang of up to 10 people burst into the Westfield centre in Shepherd's Bush, west London.
The thieves used sledgehammers to break into the complex before attacking branches of De Beers and Tiffany & Co.
Security guards and police were alerted by electronic alarms but the gang sped off in several cars at about 1.20am.
Officials at Westfield, which opened in October 2008 and houses more than 250 shops, said it was the first burglary of its kind to take place.
But it appeared to be the latest development in a long series of smash-and-grab attacks on jewellers and luxury stores across the capital.
Gangs of thieves, often riding stolen mopeds and armed with heavy tools, have plagued police and retailers.
Goods worth more than £4 million have been taken as thieves targeted Watches of Switzerland, Cartier, Dolce & Gabbana and Tiffany and Co.
In one raid on the Mozafarian store in Knightsbridge last August, gems worth more than £1 million were stolen in less than a minute.
Investigators scored a success last December when six members of one gang were jailed for a total of 22 years.
The Westfield shopping complex pays for its own private security staff and police teams under an agreement with Hammersmith and Fulham Council.
The 43-acre, £1.7 billion development boasts a 14-screen cinema, 50 restaurants, a gym, a spa and a library.
Senior officers were so concerned about the amount of crime that may take place at the centre they agreed to build extra police cells before it opened.
A Metropolitan Police spokesman said officers were alerted to a "burglary in progress" at 1.20am.
He said: "A group of up to 10 people forced access to the premises before breaking into two jewellery shops and making off with a quantity of goods."
A Westfield spokeswoman said a "quantity of jewellery" was stolen and that the centre would open as normal today.
The spokeswoman added: "Westfield is working closely with the Metropolitan Police to identify suspects and recover property.
"No-one was injured during the incident which took place whilst the shopping centre was closed and our on-site security staff responded immediately as did the local police."
* Anyone with information should contact Hammersmith Police or Crimestoppers anonymously on 0800 555 111.