The gang is accused of installing a device on the bank's computer system that allowed it to carry out the cyber theft.
The men, aged between 24 and 47, are being questioned about conspiracy to steal and conspiracy to defraud U.K. banks.
Police said cash, jewels and thousands of credit cards have been found in searches at addresses in the greater London area.
The arrests follow a failed attempt to use similar technology to rob the Santander bank last week. The same police investigators are handling both cases.
Detective Supt. Terry Wilson said one of the arrested men is the "Mr. Big" of British cybercrime.
Police suspect that in both cases a gang member posed as an engineer and installed devices on the bank's computers that allowed the suspects, in the Barclays case, to gain information used to siphon money from the bank.
A Barclays executive said the bank acted "swiftly to recover funds" after the security breach at its Swiss Cottage branch in north London in April.
"We can confirm that no customers suffered financial loss as a result of this action," said Alex Grant, the bank's managing director of fraud prevention.
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