German police are investigating the hacking of computers at the European Central Bank.
The Frankfurt-based bank said some email addresses were stolen along with other contact details.
But it stressed that no market sensitive information was accessed.
The ECB is currently running a particularly sensitive review of the eurozone’s top banks, collecting information to gauge whether they have correctly valued loans and other assets correctly, before it starts supervising them.
The hackers broke into a database storing details of people who had registered for ECB conferences, visits and other events some of which was not encrypted, the bank said.
That database, which held about 20,000 email addresses and a much smaller number of postal addresses and phone numbers, was kept physically separate from internal systems, it added.
The ECB confirmed it has contacted everyone who might have had their details stolen.
The hack attack came to light after the central bank received an anonymous email demanding money in exchange for the stolen addresses.