* About 20,000 email addresses could be affected
* ECB says no market sensitive data were compromised
* German police investigating the case (Adds number of email addresses)
FRANKFURT, July 24 (Reuters) - The European Central Bank said on Thursday its website had been hacked and some email addresses and other contact information stolen but insisted no market-sensitive data were affected.
The theft came to light after the central bank received an anonymous email on Monday night demanding money in exchange for the stolen addresses.
The hackers broke into a database storing details of people who had registered for ECB conferences, visits and other events, the bank said.
That database, which held about 20,000 email addresses and a much smaller number postal addresses and phone numbers, was kept physically separate from internal systems, it added.
"No internal systems or market sensitive data were compromised," the ECB said in a statement.
The ECB is currently running a particularly sensitive review of the euro zone's top lenders, collecting streams of data to gauge whether banks have valued loans and other assets correctly, before it starts supervising them.
German police were investigating the breach and all people who might have had their details stolen had been contacted, said the bank. (Reporting by Eva Taylor; Editing by Andrew Heavens)