Ramon Fonseca said the firm Mossack Fonseca had lodged a criminal complaint with Panamanian prosecutors on Monday over the breach.
He added that in all the reporting so far "nobody is talking of the hack, and that is the only crime that has been committed."
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In a telephone message responding to AFP questions, Fonseca said: "We have lodged a complaint. We have a technical report that we were hacked by servers abroad."
He did not specify from which country the hack was carried out.
Fonseca also rued the fact that reporting on the 11.5 million documents taken from Mossack Fonseca's computer system focused on the high-profile clients who had used the law firm to set up offshore companies to hold their wealth.
"We don't understand. The world is already accepting that privacy is not a human right," he said.
The hack has badly shaken Panama's financial services sector, which relied on discretion to do its business.
With high-profile politicians, sports stars, celebrities and a few criminals revealed to have used Mossack Fonseca to set up offshore entities, scrutiny on the small Central American nation has suddenly ramped up.
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But the law firm and the government have stressed that offshore companies are not, in themselves, illegal, and that Mossack Fonseca was not responsible for what its clients used them for.
The government, which has recently seen through reforms to get the country taken off an international list of states seen as money-laundering hubs, is mounting a fierce defense of the financial sector, which contributes seven percent of gross domestic product.
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