The offshore financial dealings of the rich and famous have been exposed by a cache of 11.5 million leaked records, an international coalition of media outlets said.
The Washington-based International Consortium of Investigative Journalism said the extensive investigation detailed the offshore holdings of a dozen current and former world leaders, as well as businessmen, criminals, celebrities and sports stars.
The allegations were published by more than 100 news organisations around the world, and the German newspaper Sueddeutsche Zeitung, which first received the data more than a year ago, said it was confident the material was genuine.
The Munich-based daily was offered the data through an encrypted channel by an anonymous source who requested no monetary compensation, said Bastian Obermayer, a reporter for the paper.
The data concerned internal documents from a Panama-based law firm, Mossack Fonseca.
Founded by German-born Juergen Mossack, the company has offices across the globe and is among the world's biggest creators of shell companies, the newspaper said.
Ramon Fonseca of Mossack Fonseca said the firm had no control of how its clients might use offshore vehicles created for them.
"We are not responsible for the actions of a corporation that we set up," he told Panama's Channel 2.
ICIJ said the law firm's internal files contain information on 214,488 offshore entities connected to people in more than 200 countries and territories, and it thought it was the biggest leak journalists had ever worked on.
Among the countries with past or present political figures named in the reports are Iceland, Ukraine, Pakistan, Saudi Arabia, Russia and Argentina.
The Guardian newspaper, which took part in the investigation, published a video on its website showing an interview with Iceland's prime minister, Sigmundur David Gunnlaugsson conducted by Sweden's SVT television.
He is asked about a company called Wintris, and responds by insisting that its affairs are above board and calling the question "completely inappropriate" before breaking off the interview.
In Russia, the Kremlin last week said it was anticipating what it called an upcoming "information attack".
Vladimir Putin's spokesman, Dmitry Peskov, told reporters the Kremlin had received "a series of questions in a rude manner" from an organisation that he said was trying to smear the Russian president.
"Journalists and members of other organisations have been actively trying to discredit Putin and this country's leadership," Mr Peskov said.
The office of Argentina's president, Mauricio Macri, confirmed the business group owned by his family set up Fleg Trading, an offshore company based in the Bahamas.
But it said Mr Macri himself had no shares in Fleg and never received income from it.
The ICIJ said the documents included emails, financial spreadsheets, passports and corporate records detailing how powerful figures used banks, law firms and offshore shell companies to hide their assets. The data spanned nearly 40 years, from 1977 through to the end of 2015.
According to the media group's website, global banks including HSBC, UBS, Credit Suisse, Deutsche Bank and others have worked with Mossack Fonseca to create offshore accounts.
"The allegations are historical, in some cases dating back 20 years, pre-dating our significant, well-publicised reforms implemented over the last few years," HSBC spokesman Rob Sherman said.
"We work closely with the authorities to fight financial crime and implement sanctions."
Panama's president said his government will co-operate "vigorously" with any judicial investigation arising from the leak.
President Juan Carlos Varela said the revelations should not detract from his government's "zero tolerance" for any illicit activities in Panama's finance industry.