Attacks by online activists who paralyzed a string of government websites have exposed serious flaws in official security systems used in Poland, experts have warned.

Activists protesting at plans they say will lead to internet censorship jammed the websites of the Polish parliament and the Ministry of Culture over the weekend by directing streams of artificially generated traffic at them.

More serious attacks were carried on Monday, the Rzeczpospolita daily noted, after hackers broke into the website of the prime minister’s office. They placed a mocking picture on it with a caption that said, “HACKED! by Polish Underground. STOP ACTA!”

ACTA is the acronym for the Anti-Counterfeiting Trade Agreement, an international copyright treaty.

Prime Minister Donald Tusk said Tuesday he had authorized Poland’s ambassador to Japan to sign the treaty in Tokyo on Thursday, alongside other EU member states.

Opponents have criticized the treaty as a clampdown on freedom of speech.

In Poland, hackers also broke into a computer used by Igor Ostrowski, a deputy minister for administration and digitization, copying data from it.

Rzeczpospolita reported that Poland’s Internal Security Agency had warned last year that the internet security of state institutions was at risk.

"It appears that the report was not taken seriously,” the paper quoted Andrzej Barcikowski, the former head of the intelligence agency, as saying.