Some 41,000 people rallied in Germany, including 16,000 in Munich and 10,000 in Berlin, against the Anti-Counterfeiting Trade Agreement (ACTA), which was negotiated between the 27-nation European Union and 10 other countries.
Many brandishing "Stop ACTA" banners and wearing Guy Fawkes masks -- a symbol of hacker-led rallies -- the mostly young protestors also braved subzero temperatures to mass in cities such as Budapest, Bucharest, Bratislava, Prague, Paris, Sofia, Tallinn, Vilnius and Vienna.
ACTA is awaiting ratification from several governments, but intense opposition led by Internet users has forced some EU states including Poland, the Czech Republic and Slovakia to freeze their ratification process.
"We see the suspension of ratification as a victory, but we cannot over-estimate it," said the vice-president of the Czech Republic's pirate party, Mikulas Ferjencik.
"We want ACTA to be stopped completely," he added.
In Sofia, more than 3,000 demonstrators marched along major downtown boulevards, booing at the buildings of government and parliament.
Shouting "No to ACTA!" and "Mafia!", they accused the government of signing the agreement secretly and without consulting the public.
In Tallinn, where about 1,500 turned out, lawmakers widened their criticism of ACTA to an attack on the country's leadership.
"Estonia's PM Ansip has often demonstrated that government decisions in Estonia are born somewhere in hidden cellars," charged lawmaker Juku-Kalle Raid, whose party governs with Andrus Ansip's Reform Party.
"The current case with ACTA only indicated that once again a decision was to be made without discussion with the people," added the lawmaker.
The European Commission meanwhile published a document detailing the negotiation process of the pact, as it sought to defend itself against accusations of opacity.
"The EU strongly denies having provided any kind of preferential access to information to any group of stakeholders," it said.
"There are also no secret protocols to the agreement and the final text is fully public and available to all citizens on the website of the European Commission," it added.
ACTA was signed last year in Tokyo, and aims to bolster international standards for intellectual property protection, for example by doing more to fight counterfeit medicine and other goods.
But its attempt to attack illegal downloading and Internet file-sharing has sparked charges that it compromises online freedom.
"I am here because I am against censorship on the Internet, against the attempts to limit the freedom of information and against corporate interests which trample on human rights," Maya Nikolova, 27, told AFP in Sofia.
Many Bulgarian musicians were also among the crowd, claiming that they rarely get copyright royalties anyway but were ready to sacrifice whatever little they do earn for the sake of Internet freedom.
One of the Vilnius rally organisers, Mantas Kondratavicius, told AFP: "Some provisions of the treaty are too ambiguous and allow different interpretations."
"If ACTA is approved, the understanding of human rights and privacy would change and there can be no way back," warned the 21-year-old.
"I don't deny that authors should be paid but that cannot be done at the expense of privacy or freedom of speech," he stressed.
Meanwhile, some on Facebook chose to protest through blood donations.
"Blood is a life-giving power, just as information and ideas are for the web. Join our symbolic way to show that sharing is not a crime but has vital importance," its organisers said.
Besides the EU, other ACTA signatories are Australia, Canada, Mexico, Morocco, New Zealand, Japan, Singapore, South Korea, Switzerland and the United States.
