WARSAW, Poland — Hundreds of people waged a street protest in Warsaw on Tuesday to protest the government's plan to sign an international copyright treaty, hours before several popular websites plan to go dark for an hour over the issue.

Poland's support for the Anti-Counterfeiting Trade Agreement, or ACTA, has sparked days of protest, including attacks on government sites, by groups who fear it could lead to online censorship.

Prime Minister Donald Tusk insisted Tuesday that his government will not give in to the protesters. He vowed that Poland will sign the international agreement, which is aimed at protecting intellectual property -- like music and books -- and products including pharmaceuticals and designer items. ACTA enjoys widespread support from the producers of music, movies and a range of goods enjoying copyright protections.

"There will be no concessions to brutal blackmail," Tusk said at a news conference.

The sites that are protesting are primarily ones that are popular with young people and carry a mix of celebrity news, jokes, funny photographs and other entertaining material.

One site, www.wykop.pl, said that "under the banner of fighting piracy and concerns about intellectual property, ACTA will limit the rights of each of us."

At the street protest, held in front of a European Union office, people carried banners that said "Stop ACTA," while some had tape over their mouths to signify their fears that ACTA will infringe on freedom of expression online.

An extremist right-wing group is planning a separate protest Wednesday to oppose ACTA.

However, an influential group representing authors and composers -- known by its Polish acronym, ZAIKS -- has thrown its support behind ACTA. ZAIKS argued that ACTA will not hurt Internet freedom but protect the rights of creators. It said that Internet piracy is now robbing artists and the state treasury of hundreds of millions of zlotys (many millions of dollars) in income.

ACTA shares some similarities with the hotly debated Stop Online Piracy Act in the U.S., which was shelved by lawmakers last week after Wikipedia and Google blacked out or partially obscured their websites for a day in protest.

In recent days, a group calling itself "Anonymous" attacked Polish government websites, leaving several paralyzed on Sunday and Monday. On Tuesday, most appeared to be working again, though the prime minister's site was unreachable. Still, Polish leaders are vowing to stick to plans to sign ACTA in Tokyo on Thursday.

ACTA has been negotiated by a number of industrialized countries that have been struggling for ways to fight counterfeiting and intellectual property theft -- crimes that cause huge losses to the movie and music industries and many other sectors.

The far-reaching agreement would cover everything from counterfeit pharmaceuticals to fake designer handbags to online piracy. The United States signed ACTA in October in Tokyo along with seven other countries: Australia, Canada, South Korea, Japan, New Zealand, Morocco and Singapore.

Critics of ACTA accuse the negotiating countries of hammering out the agreement in secret and failing to consult with the broader societies along the way.