The website of the European Central Bank (ECB) has been hacked, with personal information stolen.
The hacker demanded money for stolen data, which included contact information for people who had registered for events at the ECB, the central bank said.
The ECB said that no market sensitive data or internal banking systems were compromised.
An investigation has been launched by German police.
The hacking came to light late on Monday night when an anonymous hacker contacted the bank, an ECB spokeswoman said.
The anonymous hacker stole around 20,000 email addresses, and a smaller number of telephone numbers and addresses, relating to people who had registered for ECB conferences and visits.
People whose details were compromised have been informed, and passwords have been changed, the spokeswoman added.
The information was stolen from a database serving the bank's website that was physically separate from the bank's internal systems, the ECB added.