As many as 70 deposit boxes were opened during a raid in London's jewellery quarter, police have said.
Burglars gained entry to Hatton Garden Safe Deposit over the Easter weekend. Scotland Yard was alerted on Tuesday.
Officers said a "slow and painstaking process" of forensically examining the scene was under way.
Ex-Flying Squad chief Roy Ramm has said he "would not be surprised" if the jewellery stolen during the raid was worth as much as £200m.
The Met, which has not yet commented on the worth of the jewellery stolen, said it was "photographing the scene and recovering exhibits in meticulous detail".
It said the burglars used heavy equipment to break into a vault used mainly by jewellers and gold dealers.
A spokesman said: "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."
BBC News correspondent Daniel Sandford said: "There is still a concern that the thieves may have had some kind of inside knowledge. For example, the brand new alarm system which had just been fitted in the building had not yet been switched on."
Michael Miller, a jeweller from Knightsbridge, said he "felt sick" at the prospect of losing up to £50,000 of uninsured jewellery and watches.
He said a £5,000 watch he bought for his son might have been stolen and he felt "dismay, shock and disbelief" when he heard the news.
Gem industry expert James Riley said most businesses storing items would be insured but individuals may not.
"If you can't afford your jewellery insurance you put it in a safety deposit box which is going to cost you between £300 to £400 a year and you know it is the most secure place you can put it as your insurance company would be very happy with you putting it there," he said.
Ex-Flying Squad chief Mr Ramm told BBC Radio 4's Today programme: "There's a sort of old-fashioned audacity about it.
"The amount of money and the goods that are taken is never fully revealed... and there's a good chance that not everybody would declare.
"I would not be surprised, given where this one is in Hatton Garden, if £200m is about the amount stolen."
Norman Bean, who had a diamond ring and bracelets stored at Hatton Garden Safe Deposit, said he was told the burglars accessed the vault via a lift shaft.
"They went down the lift shaft, made a big hole in the wall, got into the safe deposit and I think they drilled into the boxes.
Hatton Garden: London's Jewellery Quarter
- Hatton Garden is situated between the City of London and the West End near Holborn
- The area is well known for its diamond and gold trade
- Hatton Garden London estimates there are about 300 businesses and more than 55 shops specialising in jewellery
- The term "jewellery quarter" dates back to medieval times when certain streets were dedicated to specific trades
"They could have been there all weekend, who knows? It's a disgrace, it's like something out of a film," added Mr Bean.
Neil Duttson, a diamond dealer, said tracing any gems stolen in the heist would be nearly impossible.
"Once diamonds have been re-cut and polished there is no geological map," he said.
"I imagine they will be sat on for six months. You can expect some cheap diamonds will be coming on the market soon."
London jewellery robberies
- In July 1987 the Knightsbridge Safe Deposit Centre suffered what is thought to be one of the largest robberies in history. Two armed robbers asked to rent a safe deposit box and, after being shown into the vault, produced guns and made off with an estimated hoard of £60m. Valerio Viccei was sentenced to 22 years
- In 2003 jewellery, cash and valuables worth an estimated £1.5m were stolen when a criminal, posing as a customer, emptied a number of boxes at the Hatton Garden Safe Deposit company