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."...MORE
