Margetic is charged with publishing a list of 102 protected witnesses on his web page from July 7 to August 2 this year. The witnesses had testified in the trial of former Bosnian Croat commander Tihomir Blaskic.
Prosecutor called two tribunal investigators as witnesses to describe how they had found the evidence on Margetic's web page, and one of the witnesses whose identity Margetic had revealed.
The witness, whose identity was protected during his testimony, said that he had been feeling unsafe and scared ever since his identity was made public.
"When I first came here to testify, I was told that we would be protected from everything," the witness said.
Prosecutor Ann Sutherland said in her opening statement that the accused had known that the witnesses were protected and that he had knowingly violated the tribunal's orders.
The documents Margetic obtained as part of another court case were clearly marked confidential, she stressed, adding that he had published the list under the banner "Exclusively - A Confidential List of Hague Tribunal Witnesses Provided by Carla del Ponte's Aide".
The prosecutor said that some of the witnesses had suffered the mental consequences of publication of their identities, were living in fear for their own safety and the safety of their families, and were no longer willing to cooperate either with the Hague tribunal or with domestic courts.
Margetic's defence attorney Veljko Miljevic said that his client had not been aware he was violating the tribunal's order because the trial chamber in the Jovic case decided on July 11 to make the list public, and that the tribunal made a new decision on August 22 declaring the list classified again.
Margetic insisted he had received no letters from tribunal officials saying he was obliged to respect the confidential data, but only a compact disc containing the names of protected witnesses. He also said he had found somewhere on the Internet the tribunal's decision dated July 11 making public the list of protected witnesses.
The accused also claimed he had not given a statement to Hina on August 2 in which he explained that he believed he had been summoned by the Zagreb County Court for publishing a confidential list of protected witnesses.
Margetic said several times during his testimony he never meant to discredit or offend the Hague tribunal.
If found guilty, he faces a maximum of seven years in prison or a fine not exceeding 100,000 euros.