WARSAW — Nearly 100 Polish websites shrouded their pages in black early Tuesday to protest against Warsaw's plan to sign a multilateral anti online piracy accord this week.

The Anti-Counterfeiting Trade Agreement (ACTA) aims to create international standards for intellectual property protection, but some groups oppose it as limiting Internet freedoms.

Poland, which joined the EU in 2004, has committed to signing ACTA on Thursday.

Popular Polish websites including satirical joemonster.org and demotywatory.pl donned black shrouds for several hours Tuesday morning, Poland's TVN24 news channel reported.

Meanwhile, a notice on a silent street protest in Warsaw planned for Tuesday afternoon, organised by "Stop ACTA" campaigners on Facebook, attracted more than 38,000 "like" tags.

The fresh wave of protest comes after weekend anti-ACTA cyber attacks by the groups Anonymous and Polish Underground took down official websites belonging to Poland's president, prime minister, parliament and foreign and culture ministers.

The prime minister's website (www.kprm.gov.pl) was still down Tuesday, with links directing users to his official Blip, Facebook and Twitter pages.

An attack early Monday morning left the message "Hacked by the Polish Underground: Stop ACTA" appear on it.

Under a vast Internet anti-piracy campaign launched over 18 months ago, US authorities have seized more than 350 website domain names, including a spectacular global swoop on file-sharing site Megaupload.com.

But US congressional leaders put strict anti-online piracy legislation on hold following a recent wave of protests led by Google and Wikipedia denouncing the bills as a threat to Internet freedom.