CopenhagenHackers compromised 8.8 million records, more than Denmark has residents.
The records hold names, addresses and national registry numbers, plus church membership and legal-capacity restrictions.
Initial findings suggest the hackers used the legitimate login credentials of a local company.
Hackers have not been identified, and the digital affairs ministry says the total includes dead and emigrated people.
Digital affairs minister Christina Egelund said authorities will investigate the full extent.
Sources: Deutsche Welle