Thousands of young Poles hit city streets across the country in a mounting wave of off-and-online protest against a government decision to sign an international anti-online piracy accord.
The Anti-Counterfeiting Trade Agreement (ACTA), which Poland's centrist Prime Minister Donald Tusk has vowed to endorse, aims to create international standards for intellectual property protection.
However, internet groups including global hacker collective Anonymous oppose it as limiting online freedoms.
Poland, an ex-communist state which joined the European Union in 2004, has committed to signing ACTA on Thursday, local time.
A protest by thousands organised largely via Facebook in the central Polish city of Kielce yesterday turned violent when some protesters trashed cars and attacked police, the commercial TVN24 news channel reported.
Protesters also turned out for anti-ACTA rallies in Wroclaw, Szczecin, Olsztyn and Bialystok.
Online protest pages on Facebook have attracted more than 300,000 supporters, while an anti-ACTA online petition had drawn about 130,000 signatures by Wednesday evening.
Protesters are upset Tusk's government pushed ahead with ACTA after meetings with record companies and commercial media, but held no public consultations with online rights groups.
Thousands of people rallied in Warsaw on Tuesday and over 100 Polish websites shrouded their pages in black to protest Poland's endorsement of ACTA.
Weekend anti-ACTA cyber attacks by Anonymous and another hacker group called Polish Underground took down the official websites of Poland's president, prime minister, parliament and foreign and culture ministers.
The prime minister's website came back online Wednesday morning. Tusk's chancellery also admitted Wednesday it had deleted over 5,000 ACTA-related comments on its Facebook page and had blocked some of their authors, Poland's RMF24 news radio reported.
Social media activists on Wednesday were planning more anti-ACTA street protests for later this week.