The attack was most probably carried out by members of the hacker group called Anonymous, who had announced that they would hack it ahead of Saturday's protests against the signing of the controversial international Anti-Counterfeiting Trade Agreement (ACTA) which regulates the protection of intellectual property rights.

The President's website was also attacked on Thursday, at about 2240 hours, when it was unavailable for about an hour. The Anonymous group told media that they had carried out the attack. The President's Office later said it would take the necessary steps to protect its website.

President Ivo Josipovic said on his Facebook profile on Friday that it was necessary to protect intellectual property and stop piracy, adding that the Internet must remain a free platform for communication, for the further development of creativity, economy, society and democracy, and for exchanging views.

"Piracy, especially if it is for commercial purposes, needs to be stopped. In that way we protect creativity and encourage economic development," Josipovic wrote.

The Croatian Music Institute on Friday condemned the hacker attack on the President's website, describing it as "an unacceptable show of force" and an attack on freedom of expression on the Internet.

Protests against ACTA are planned throughout Europe on Saturday, including in Zagreb and several other cities in Croatia. Protesters fear that the agreement might jeopardise freedoms on the Internet.

ACTA was signed on October 1, 2011 by Australia, Canada, Japan, Morocco, New Zealand, Singapore, South Korea, and the United States. In January 2012 they were joined by the European Union and 22 of its member states, while some postponed signing it. The Croatian government has announced that there will be a public debate on the matter.