Emails, pdf files and other records from the internal database of Mossack Fonseca, a Panama-based law firm. They are said to cover a period from the 1970s to the spring of 2016.
The so-called Panama Papers are said to provide data on around 214,000 offshore companies. The BBC alleges the documents show how the law firm has helped clients launder money, dodge sanctions and evade tax.
Mossack Fonseca said it had operated "beyond reproach" for 40 years and had never been charged with criminal wrongdoing.
German newspaper Sueddeutsche Zeitung first received the data more than a year ago. It said the information was offered by an anonymous source who requested no cash compensation.
More details are expected to be released in the coming days although the International Consortium of Investigative Journalists, based in Washington, said the offshore holdings of 12 current and former world leaders are detailed in the investigation.
It added criminals, celebrities, sports stars and businessmen are also included.
More than 300 companies in Ireland have been linked to the so-called 'Panama Papers' release.
Across the water reports also suggest six British peers, three Tory ex-MPs and "dozens" of UK political party donors are also among those identified as holders of offshore assets.
Iceland's prime minister Sigmundur David Gunnlaugsson has been questioned about claims that he failed to declare an interest in his country's bailed-out banks. He is facing calls for his resignation.
Tax avoidance involves companies and people using legal ways and following the rules to reduce their tax bill, which on a small scale can include using a tax-free Isa to save money.
In contrast, tax evasion is an offence and involves illegal ways of paying less tax than required.
It is claimed the leaked papers show attempts to use offshore companies to conceal the identities of the actual company owners.