Hacker groups LulzSec and Anonymous announced on Monday that they had hacked into the website of Italy's cyber police and published classified information online in an attack dubbed "Operation Italy".
The police unit admitted it had been hacked but did not confirm or deny that any information had been stolen.
The attack "revealed some of the most important and most secret reports of the cyber police on illegal and immoral practices," hackers said on a blog.
The reports "will show abuses committed in Europe by the CNAIPIC (National Centre against Cyber Crime and for the Protection of Critical Infrastructure)."
They said the leaks would be published on anonops-ita.blogspot.com.
The information published so far is 8 megabytes of information on the Italian government's relations with Australia, Belarus, Egypt, Russia and Ukraine.
"We are working to try to understand the scale of what happened," CNAIPIC said in a statement. "We have to verify if documents have been taken and which ones."
Emanuele Fiano, a lawmaker from the opposition Democratic Party, said: "This cyber attack against the police task force is of an unprecedented gravity."
He said he would propose new laws to crack down on hackers "in order to improve this fundamental aspect of our country's security."
CNAIPIC was set up in 2008 to crack down on cyber crime.
The attack comes after Anonymous and LulzSec promised on Thursday to continue their struggle against companies and governments, two days after a police bust.
Instead of maintaining a low profile after the arrest of 16 people in the United States and five in Europe in an inquiry led by the FBI against Anonymous, the hackers said they would widen their struggle.
A month ago LulzSec announced an end to its hacking campaign after an arrest in Britain although it said it was counting on sympathisers to continue.
The group first became known for its attack on Sony and Nintendo video games before attacking the CIA and the FBI. It also hacked into websites belonging to Rupert Murdoch's News Corp.
Explore further: Japan court orders Facebook to reveal revenge porn IP addresses