LONDON (Reuters) - British police said on Friday they had arrested eight people including the suspected mastermind of a cyber-theft in which criminals infiltrated the computer system of a branch of Barclays bank and stole 1.3 million pounds.

Last week police arrested 12 men allegedly planning to steal millions of pounds from a London branch of Santander UK with the same device - a "keyboard, video and mouse" (KVM) that allows legitimate workers to access computers remotely.

The Barclays gang are accused of stealing the money in April after one of them posed as an IT engineer to attach the KVM to a computer at the bank's local Swiss Cottage branch in London.

"This was a highly organised criminal network with each individual filling a specific role," said Detective Superintendent Terry Wilson, head of London Police's Central e-Crime Unit.

"All criminal networks have a head and we very much believe we have now apprehended our ‘Mr Big' as part of this operation. We believe this has put a major dent in this alleged criminal network's ability to commit cyber-crime in this country."

The KVM enabled the perpetrators to remotely siphon off cash from customers' accounts and transfer it to bank accounts under their control, police said.

London detectives said the scam was a new and growing type of cyber crime that was low-risk but gave high rewards. Wilson said they were working with banks to boost safety measures.

"Those responsible for this offence are significant players within a sophisticated and determined organised criminal network, who used considerable technical abilities and traditional criminal know-how to infiltrate and exploit secure banking systems," said Detective Inspector Mark Raymond.

Police said the eight men, aged between 24 and 47, had been arrested on Thursday and Friday. Searches at properties across London yielded cash, jewellery and drugs as well as thousands of credit cards, and personal data.

Barclays were able to recover a significant amount of the stolen cash, the police said.

(Reporting by Sarah Young and Michael Holden; Editing by Kevin Liffey)