A group calling itself the Armada Collective demanded the bitcoin ransom after staging its first attacks, and then threatened a full collapse of the unnamed banks websites if they refused to pay up.
These initial attacks took the form of a distributed denial of service - flooding the banks websites with requests so that they crashed under the strain. On Thursday, they succeeded in disrupting electronic transactions at all three banks for a short period, but customer information was protected.
Cyber-experts from the Greek central bank and the police electronic crime unit were continuing to monitor the banks computer systems, a central bank official added.
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