Detectives in Britain investigating a heist on a safe deposit firm believe up to 70 boxes were opened during the burglary.
A gang of burglars escaped following the raid at Hatton Garden Safe Deposit company in London's famous Hatton Garden jewellery quarter.
The incident happened over the Easter weekend but police were only alerted yesterday.
The raiders used heavy equipment to break into a vault containing several deposit boxes, mainly used by jewellers and gold dealers.
A Metropolitan Police spokesman said: "This is a slow and painstaking process involving forensic examination, photographing the scene and recovering exhibits in meticulous detail in order to preserve the evidence.
"Officers anticipate this process to take approximately two days."
The spokesman said it is believed that approximately 60-70 safety deposit boxes were opened during the burglary.
"Officers are working closely with Hatton Garden Safe Deposit Ltd to establish the identities of those affected.
"Police will be contacting victims directly as and when they are identified," he said.
On its website, Hatton Garden Safe Deposit Ltd said it was founded in 1954 and was one of the first companies in Britain to offer safe deposit boxes.
One jeweller spoke of his "extreme shock" today and said he feared that a £5,000 watch he bought for his son on the day he was born might have been stolen.
Michael Miller, a jeweller from Knightsbridge, London, said he "felt sick" at the prospect of losing up to £50,000 of jewellery and watches during the burglary.
He revealed that his goods - like those of many with deposit boxes there - were uninsured.
"I can't believe this has actually happened," he said.
"If you look at their website, they say they are the safest place around. There is a double-door entry and a locked system to go in. You have to go through two doors to get in the place and then get into the vault.
Local media said it could be the Britain's biggest-ever heist.
Police gave no details of what was stolen but the Sun newspaper said the haul could be as much as £200m (€275m), dwarfing the amounts taken in Britain's previous biggest heists.
Britain's record heist was carried out at a Securitas cash depot in Tonbridge, southeast of London, in 2006 when robbers stole almost £53m in cash.
In 2000, police foiled a daring attempt to steal £350m worth of diamonds from a display at London's Millennium Dome in what would have been the world's biggest ever robbery.
