In total, 32 people were arrested after American Express alerted the US authorities to its suspicions of fraudulent activity by two employees.
Most of the arrests, which followed a five-month investigation, were in North America after a swoop in several US cities, Montreal and London.
US investigators suspect that the two employees were tapping into the company's computer system and giving confidential information about customers to accomplices across the US, Canada and the UK.
A Scotland Yard spokesman said yesterday that two men and two women had been arrested under section seven of the Forgery and Conterfeiting Act 1991.
"As part of a joint operation between the Metropolitan Police and law enforcement agencies throughout the US, six search warrants were executed at addresses in London," the spokesman said.
He declined to disclose the identities or the nationalities of the four people arrested but said the inquiries were in connection with counterfeit credit cards.
The four have been released on police bail until 15 January pending further investigations.
The allegations of fraud affected 37 American Express accounts in the US. American Express said it had protected the accounts and reissued cards.
"By working closely with the Secret Service to isolate and monitor the activities of these employees, we were able to help an investigation that has made an important impact on a fraud ring operating within the card industry and minimise any future activity at American Express or elsewhere," American Express said in a statement.
In Washington, the US Treasury Department said the 32 people arrested were indicted for conspiracy to commit access device fraud and wire fraud.
Searches had produced many American Express credit cards and credit histories of American Express customers, cash and forged documents, the Treasury Department said.