Poland’s largest opposition party said Thursday it would seek a referendum on whether the country should ratify an international copyright treaty that has brought over 15,000 out on the streets to protest at a deal they fear will lead to internet censorship.

Jaroslaw Kaczynski, leader of the conservative-nationalist opposition Law and Justice (PiS) party, said: "We will submit a motion in parliament to hold a referendum about Poland signing, accepting, this very highly controversial international agreement."

Poland, along with 21 other EU countries, signed the international Anti-Counterfeiting Trade Agreement (ACTA) in Tokyo on Thursday. The treaty will need to be ratified by the Polish parliament and signed by the president.

Opponents of the agreement, which aims to protect international intellectual property rights and fight online piracy, launched a string of attacks on Polish government websites earlier this week, jamming the portals of the Polish parliament, the prime minister’s office and the Ministry of Culture.

On Thursday evening, five people were detained after bottles were thrown at police in the western city of Poznan following a rally at which several hundred demonstrated outside the ruling Civic Platform party’s local headquarters.

Over 15,000 protesters in total have demonstrated in cities across Poland in recent days, claiming the international treaty will limit freedom of speech.