Police thwarted an "audacious" cyber gang from stealing millions of pounds after they remotely took control of a bank branch's computers.
One of the plotters posed as an engineer to fit a computer in a branch of Santander with a "keyboard video mouse" device that allowed them to transmit its desktop contents.
Using the gadget, the gang remotely took control of all the computers at the branch in Surrey Quays shopping centre in south east London, but the Spanish bank said they were unable to steal any money.
Officers yesterday arrested 11 men aged between 23 and 50 in Hounslow, west London. A 34-year-old was arrested in Victoria, west London.
Searches were carried out at addresses in Westminster, Hounslow, Hillingdon, Brent and Richmond, and also in Slough, where property has been seized.
Detective Inspector Mark Raymond, from Scotland Yard's Police Central e-crime Unit (PCeU), said:
"This was a sophisticated plot that could have led to the loss of a very large amount of money from the bank, and is the most significant case of this kind that we have come across.
"The PCeU is committed to tackling cyber-crime and the damage it can cause to individuals, organisations and the wider economy."
Santander said that no member of staff was involved in the attempted raid.
A spokesman said: "Like all high street banks, Santander works very closely with the police and other authorities to help prevent fraud.
"Through this co-operation, Santander was aware of the possibility of the attack connected to the arrests. The attempt to fit the device to the computer in the Surrey Quays Branch was undertaken by a bogus maintenance engineer pretending to be from a third party.
"It failed and no money was ever at risk. No member of Santander staff was involved in this attempted fraud. We are pleased that we have been able, through the robustness of our systems, to prevent the fraud and help the police gather the evidence they needed to make the arrests."
The bank branch pulled its shutters down and queues of worried customers used the ATMs outside to check whether all funds were still in their accounts.
They looked relieved after finding their money was still there, but still expressed concern about cyber crime.
Anita Barnett, 79, a pensioner from Bermondsey, said: "I had my purse stolen in a shop a few years back but this is much worse. People only carry petty cash in their purse but have got their whole pensions in those accounts.
"This is much more serious and should be stopped."
One woman, who identified herself as Sally, 38, from Rotherhithe, said she would be taking her money out and closing her account.
"A couple of years ago I had my card cloned at a hole in the wall and £600 stolen.
"I'm definitely thinking of moving bank. It doesn't feel safe to leave all my money in there any more after this."
Robert Peterson, 54, also from the Rotherhithe area of south-east London, said he had come to the bank after hearing news of the attempted theft on the radio.
"It's very worrying. I know it's progress but people want to know if their money is secure with these computer systems," he said.
"At the end of the day, it's other people looking after our money."