The attempted multi-million pound theft at Santander involved fitting a keyboard video mouse (KVM) device to a computer within the bank's Surrey Quays branch, the Metropolitan Police has said.
This would allow the suspects to view the desktop contents of the bank computer and take control of it from a remote location.
The Met said: "Yesterday's time-critical, dynamic response was achieved by working in partnership with the banking sector, thwarting a very significant and audacious cyber-enabled offence."
The 12 men arrested are being held at a London police station following searches at a number of addresses across the capital.
Twelve men have been arrested over an alleged plot to take over Santander Bank's computer to steal millions of pounds from the bank.
The group are accused of fitting a device to a computer within the bank's Surrey Quays branch in south-east London and attempting to remotely control the bank's main computer.
Eleven men, aged between 23 and 50, were arrested in Hounslow, while a 34-year-old man was held at Vauxhall Bridge Road.
The arrests yesterday were the result of a long-term investigation by the Metropolitan Police's Central e-Crime Unit, which described the alleged plot as "audacious".