Email addresses and contact data have been stolen from the European Central Bank's (ECB) website.
The information was taken from a database that handles conferences, visits and other events at the ECB, and no internal system or market data was compromised.
A spokesperson said: "The ECB takes data security extremely seriously. German police have been informed of the theft and an investigation has started."
The bank was alerted to the attack when an anonymous hacker contacted them seeking money for the data.
"While most of the data were encrypted, parts of the database included email addresses, some street addresses and phone numbers that were not encrypted," the spokesperson added.