The trove of 11.5 million documents from Panama-based law firm Mossack Fonseca was obtained from an anonymous source by Germany's "Süddeutsche Zeitung" newspaper.
The documents, dubbed the "Panama Papers," show how the company helped its clients evade tax and launder money. The files also link current and former leaders, including Egyptian autocrat Hosni Mubarak, Libya's former leader Moammar Gadhafi, and Syrian President Bashar al-Assad to illicit financial dealings.
The daily Süddeutsche shared the unprecedented leak with the Washington-based International Consortium of Investigative Journalists (ICIJ) and other news outlets, including the BBC and "The Guardian." Their findings were published Sunday after a year-long investigation into the documents.
"2.6 terabytes of data, 11.5 millions documents, and 214,000 shell companies: The Panama Papers are the largest data leak journalists have ever worked with," Süddeutsche tweeted.
Gerard Ryle, director of the ICIJ, said the documents covered the day-to-day business at Mossack Fonseca over the past 40 years.
"I think the leak will prove to be probably the biggest blow the offshore world has ever taken because of the extent of the documents," he said.
Panama's Mossack Fonseca, one of the world's most secretive offshore law firms, told the ICIJ it had operated beyond reproach for decades to help clients launder money and skirt paying tax. The company denied any wrongdoing, and said it "does not foster or promote illegal acts," and carries out due diligence on its clients.
Using offshore companies is legal, but the cache of documents shows that banks, law firms and other offshore entities often fail to comply with laws requiring them to make sure their clients aren't involved in tax evasion or criminal enterprises, the ICIJ said.
Rich and powerful
The papers reveal that around 140 politicians from around the world have been using offshore tax havens - among them 12 national leaders. Other individuals mentioned in the files included drug traffickers, celebrities and sports stars.
Ukrainian President Petro Poroshenko, Iceland's prime minister, Sigmundur Davíð Gunnlaugsson; Pakistani Prime Minister Nawaz Sharif; and Alaa Mubarak, the son of Egypt's former president, are all known to have offshore wealth. Russian President Vladimir Putin isn't mentioned in the documents, but the ICIJ said his associates "secretly shuffled as much as $2 billion (1.7 billion euros) through banks and shadow companies."
The files also reveal that the families of at least eight current and former members of China's supreme ruling body, including President Xi Jinping's brother-in-law, have been found to have hidden wealth.
The name of soccer player Lionel Messi also makes an appearance in connection with a shell company he owned with his father in Panama.
Twenty-three of Mossack Fonseca's clients were found to have had sanctions imposed on them for supporting the regimes in North Korea, Zimbabwe, Russia, Iran and Syria.
The company was founded in 1977 in Panama by German immigrant Jürgen Mossack and Ramon Fonseca. It is one of the world's top creators of shell companies - structures that can be used to hide ownership of assets. The firm operates around the world in tax havens including Switzerland, Cyprus and the British Virgin Islands.