Burglars have used heavy cutting equipment to break into safety deposit boxes in a vault in London's jewellery quarter.
The raid is thought to have happened over the Easter bank holiday weekend at the Hatton Garden Safe Deposit company.
Officers from the Met's Flying Squad were called to the area at 08:10 BST.
In a statement, the Met Police said: "A number of safety deposit boxes have been broken into". They said inquiries were continuing.
The Hatton Garden area is well known for its diamond and gold trade.
BBC News correspondent Ben Ando, who is at the scene, said: "What is not clear at the moment is when the thieves actually broke in or how they broke in.
"My understanding is that they did not come in through the front door, that's because security did check during the weekend that nobody had come in through the front door and found it to be locked shut."
He said a lot of the safety boxes could contain diamonds that have not been set into jewellery.
Diamond jewellery expert Lewis Malka, who works in the area, said the haul was likely to amount to "hundreds and hundreds of thousands of pounds".
He said: "Most of the people who have got safety deposits there are people in the trade."
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 hand 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