The protests came a week after about 2,000 people marched in the Slovenian capital, Ljubljana, and two weeks after thousands of people in Poland protested against ACTA when the country and 21 other European Union states, including the UK, signed the agreement.
Following the first protests in Poland, the country's prime minister, Donald Tusk, said he would hold off plans to ratify the agreement, admitting that the negotiation process "did not involve sufficient consultation".
In the latest round of marches, the biggest protests were held in Germany, Poland and the Netherlands. Around 200 protesters gathered in central London and there were demonstrations in Edinburgh, Glasgow and other UK cities, according to the BBC.
Saturday's London demonstration was supported by the Open Rights Group, which alleges that ACTA negotiations were carried out in secret by EU bureaucrats.
UK-based privacy campaigner Big Brother Watch has called for a parliamentary debate on ACTA, also arguing that the treaty had been signed in secret.
Germany and the Netherlands are among the European states that have not yet signed the agreement, along with Estonia, Cyprus and Slovakia.
ACTA is aimed at improving the enforcement of intellectual property rights (IPR) by setting international standards for dealing with copyright infringements. But critics believe it will lead to censorship of the internet.
ACTA’s supporters insist the agreement will not alter existing laws and will instead provide protection for content creators in the face of increasing levels of online piracy.
The UK's Intellectual Property Office maintains that ACTA should not mean new laws relating to internet use.
Signing ACTA is important for the UK, the IPO said, as it will set an international standard for tackling large-scale infringements of intellectual property rights through the creation of common enforcement standards and more effective international cooperation.
The treaty cannot be enacted before it is ratified by the European Parliament after a debate scheduled for June.
This story was first published by Computer Weekly