Jewellers spoke of their shock on Wednesday and raised concerns about security at the building, the centre of the UK’s diamond trade.
Some 70 deposit boxes at the store, mainly used by Hatton Garden jewellers, are thought to have been targeted.
Scotland Yard has not commented on the value of items stolen but one jeweller said some of the boxes were worth up to £2m each.
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.
“At this stage 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.”