The Metropolitan Police's Police Central e-Crime Unit (PCeU) has arrested two teenagers in an international cybercrime investigation.
The two males, aged 17 and 18, are being held in a central London police station.
They were arrested on suspicion of encouraging or assisting crime, unauthorised computer access, and conspiracy to commit fraud.
The arrests were made as part of a wider, eight-month investigation by the PCeU into an 8,000-member global cybercriminal forum.
Detectives have recovered more than 65,000 compromised credit/debit card numbers, a potential industry loss of £7.9m.
They found evidence that the forum is facilitating the theft and sale of personal information, credit and debit card fraud, and the exchange of malicious software.
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The forum is also providing tutorials on how to commit such offences, including how to evade and frustrate law enforcement activity.
"The arrests are an example of our increasing effort to combat online criminality and reduce national harm to the UK economy and public," said detective chief inspector Terry Wilson, of the PCeU.