The United States and other proponents of ACTA deny that it will be invasive. They argue that protecting intellectual property rights is needed to preserve jobs in innovative and creative industries. The online piracy of movies and music costs U.S. companies billions of dollars every year.
Washington also vows that individuals would not be monitored online and that ACTA would instead target companies that profit from using pirated products like software.
"Civil liberties would not be curtailed," says the Office of the U.S. Trade Representative, which signed ACTA in October.
But opponents say the agreement is worded so vaguely that it is unclear what would be legal and what not. Some people fear they could be prosecuted for, say, mixing home video footage with a Lady Gaga song and putting it on YouTube to share with friends.
"Because it's unclear what is allowed, people will limit their creativity," said Anna Mazgal, a 32-year-old Polish civil rights activist. "People could censor themselves out of fear because it's so vague."
Many opponents also fault ACTA for putting commercial values like profit above rights like freedom of expression.
"It's not surprising that European citizens are taking to the streets in the thousands to protest against an agreement that puts rightsholders' private economic interests ahead of their fundamental rights," said Gwen Hinze, the international intellectual property director with the Electronic Frontier Foundation, a San Francisco-based group that defends civil liberties on the Internet.
All the uproar has put ACTA's supporters on the defensive, at least for now.
The agreement has already been signed by the United States, Japan, South Korea and about 20 other countries.
But some governments which have signed it now say they won't ratify it, including Poland, Slovenia and Bulgaria. The Czech Republic says it needs to analyze the matter before deciding. A key test will come in the summer when the European Parliament will vote on it.
Germany says it supports ACTA as a way of defending intellectual property rights, but has promised to clarify doubts about it before signing it. Thousands protested last Saturday against ACTA across Germany, where data protection has long been a widespread concern and officials have clashed with Internet giants such as Google and Facebook over privacy issues.
The Slovenian ambassador to Japan, who signed it in Tokyo last month on behalf of her nation, later apologized, saying she had not understood at the time how it could limit freedom "on the most significant network in human history."
"I signed ACTA out of civic carelessness," Helena Drnovsek Zorko wrote on her blog.
Associated Press writers Geir Moulson in Berlin and Jovana Gec in Belgrade contributed to this report.
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