Members of Iceland's political opposition called for Prime Minister Sigmundur David Gunnlaugsson to resign on Monday following a claim that he and his wife secretly channelled funds to an offshore tax haven in the Caribbean.
Gunnlagusson "owes his people to leave immediately," said former prime minister Johanna Sigurdardottir, who served after the 2008 global financial crisis caused three of Iceland's main banks to collapse.
A reported leak of more than a 11 million documents that originated at the Panama-based law firm Mossack Fonseca implicated Gunnlaugsson and several other world leaders and their associates.
The so-called Panama Papers, which disclosed the activities of 214,000 purported shell companies, were obtained by the Munich-based daily newspaper Sueddeutsche Zeitung among other news outlets. Sueddeutsche said the employee who leaked the documents was risking his life.
The documents reportedly showed that in 2007 Gunnlaugsson and his wife bought an offshore company based in the British Virgin Islands. Upon becoming a member of parliament in 2009, Gunnlaugsson reportedly sold his 50-per-cent stake in the company, Wintris Inc, to his wife for one dollar.
More than 8,000 people signed up on Facebook to protest outside Iceland's national parliament on Monday. An online petition for Gunnlagusson to resign gathered more than 22,000 signatures.
Australian tax authorities said Monday that they were investigating about 800 individuals implicated in the Panana Papers.
"Taxpayers can't rely on these secret arrangements being kept secret. We will act on any information that is provided to us," said Michael Cranston, deputy commissioner of the Australian Tax Office.
Austrian Chancellor Werner Faymann called for a thorough investigation into the purported shell companies and for Europe to develop a collective solution to the problem of tax evasion.
"There needs to be clear answers if wealthy individuals can use methods like these to defraud states of their rightful income," Faymann said Monday.
France, India and Israel also announced Monday that they would investigate the revelations and go after those evading tax.
The Italian partner for the journalistic expose, L'Espresso newspaper, said Mossack Fonseca had "about 1,000" Italian clients.
Italy's second-largest bank, UniCredit, and the chairman of Italian national airline Alitalia, Luca Cordero di Montezemolo, were among the Italian names associated with the firm, L'Espresso reported.
The European Commission on Monday would not comment specifically on the allegations against Ukrainian President Petro Poroshenko, but a spokesman said the European Union's executive is closely monitoring what progress Kiev makes on combatting corruption.
The revelations come days before the Netherlands was due to vote on a referendum for the EU to have closer political and economic ties with Ukraine. Kiev has pledged to undertake democratic reforms in return for EU support.
"The Panama Papers leak reveals how the international finance system is used to allow the rich, powerful and corrupt to launder and hide stolen assets. Nothing short of public transparency about corporate ownership can stop the rot," the Transparency International advocacy group said in a statement.
A top Russian anti-corruption official on Monday suggested that the Panama Papers leak was used for Western media "attacks" against the country and its president, Vladimir Putin.
"The number of these informational attacks against the president of Russia and the simplicity of their fabricated subjects are like a needle injecting poison numerous times in the hope that at least one dose will work," Irina Yarovaya, head of the anti-corruption committee in Russia's lower house of parliament, said in comments carried by state news agency TASS.
Media reports based on the Panama Papers said reputed members of Putin's inner circle had used secret offshore deals to hide billions of dollars.
Putin was not directly named in the leak, and the alleged secret deals did not appear illegal. However, media reports speculated that the deals enabled Putin's confidants to hide fortunes made possible through his patronage.