Hackers broke into Denmark's national population register without permission, the Danish ministry for science, education and digital affairs says. Personal data on 8.8 million people was leaked.
The register holds names, addresses and social security numbers. Data on dead people and on those who moved abroad was also exposed.
Minister Christina Egelund called it "a serious security incident." The investigation is still at an early stage, and the ministry has told people to stay alert.
The CPR system holds records on 11 million people, including those who have died or moved abroad, the ministry says. Denmark's population is about 6 million.
A full security check of the register is now underway.