The Norwegian parliament has said it has now halted what had been a "significant cyber-attack" on it.
Director Marianne Andreassen said the email accounts of several members and staff of Norway's main opposition Labour Party had been hacked.
"We take the matter very seriously," she said.
"And we have given full attention to analysing the situation to get an overall picture of the incident and the potential extent of the damage."
The Norwegian National Security Authority said it had been "assisting parliament with analysis and technical assistance" for several days.
In January, the personal details of hundreds of German politicians, including Chancellor Angela Merkel, were stolen and published online.
And last year, Australia's cyber-intelligence agency blamed China after hackers tried to break into the Australian parliament, something Chinese officials denied.