Thousands of Icelanders late Monday gathered outside the parliament in Reykjavik to call on Prime Minister Sigmundur David Gunnlaugsson to resign, following a claim that he and his wife secretly channelled funds to an offshore tax haven in the Caribbean.
Some among the crowds banged drums, others waved placards decrying alleged corruption and calling the premier a liar.
In parliament, opposition parties filed a motion of no confidence, but it was not clear when it would be voted on.
The premier rejected calls to resign or call early elections.
"The government has worked on many good things and it is best for the nation ... that it manages to finish its work and then the voters will judge our work," Gunnlaugsson said in an interview on Iceland's Channel 2.
He has been prime minister since 2013. Elections are not scheduled until 2017.
A reported leak of more than 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.
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 1 dollar.
"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.
Many other countries have been implicated in the Panama Papers disclosures. France, India, Israel, Austria, Australia and the Netherlands were among the countries that announced they would investigate the revelations.
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.
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.
Government opposition in Pakistan has demanded an investigation into Prime Minister Nawaz Sharif after his daughter and two sons were implicated in the scandal.
In Washington, White House spokesman Josh Earnest refused to comment on the specific allegations, but said: "I can tell you that the United States continues to be a leading advocate for increased transparency in the international financial system, and in working against illicit financial transactions and in fighting corruption."
He said increased transparency "allows us to rout out corruption and to fight efforts to get around US sanctions" against a number of countries.
"We're going to continue to be leading advocates for that kind of transparency, and there will continue to be large groups of national security professionals at the Department of Justice and at the Department of Treasury who will continue to be focused on these issues."