Hi-tech crime police are questioning 19 people suspected of orchestrating a multimillion-pound attack on British bank accounts.
Up to £6 million has been taken from online accounts in just three months by a gang of computer hackers.
They used a virus known as "zeus" to infect computers and capture the passwords and other sensitive details of banking customers.
Their money was then transferred into bogus accounts created by the crooks to help them launder the profits.
Detective Chief Inspector Terry Wilson, of the Metropolitan Police, said the amount of money stolen is likely to "increase considerably" as the investigation continues.
He said: "We believe we have disrupted a highly organised criminal network, which has used sophisticated methods to siphon large amounts of cash from many innocent peoples' accounts, causing immense personal anxiety and significant financial harm - which of course banks have had to repay at considerable cost to the economy.
"Online banking customers must make sure their security systems are up to date and be alert to any unusual or additional security features requested which is at variance with their normal log-on experience.
"Greater public awareness and education will make it harder for personal details to be compromised and for this type of fraud to be carried out."
Officers from the Met's Central E-Crime Unit arrested 15 men and four women aged between 23 and 47 at addresses across London in dawn raids on Tuesday.
They were questioned on suspicion of fraud, offences under the Computer Misuse Act and money laundering.