"I will not comment on documents which might have been leaked from an investigation. I never do it and I don't think that what is happening is good, even when the HDZ is concerned," he said, adding that his party had been warning about problems in the financing of election campaigns for years, including in 2005 and 2007.

"Some took part in it very much, and those who pretend not to have seen anything, they can't see anything, and such people cannot lead the state."

Asked if Prime Minister Jadranka Kosor should step down if an indictment is filed against the HDZ as a legal person, Milanovic said he was neither the public attorney nor a policeman and that he did not want to comment on the matter.

Party finances are a serious matter, we are indeed not talking about billions of kuna as is the case with some public companies, but they do require a special relationship between political parties and citizens, and citizens must be able to trust parties, said Milanovic.

The leader of the strongest opposition party was speaking after a commemoration of the 10th anniversary of the 11 September terrorist attacks on the United States, held at Zagreb's Mimara Museum.

Milanovic also said that a campaign for the forthcoming parliamentary elections had already begun and that it was not good in terms of its substance. "It is a campaign of the red and the black, which is bad and ugly," said Milanovic, noting that it was good that all parties would have to keep to the legal limit for election costs of HRK 15 million, which is the maximum amount a single party list may spend on elections.

The SDP leader would not comment on current debates between President Ivo Josipovic and PM Kosor.