British Prime Minister David Cameron led officials from dozens of nations in pledging Thursday to "substantially reduce corruption and bribery in all its forms" following an anti-corruption summit.
"Today the world has come together... [in] the biggest demonstration of political will that we have seen for many years," Cameron told reporters after officials from some 40 nations, including 11 heads of state, attended the summit.
But global anti-poverty group Oxfam said the summit had delivered an "underwhelming response to the challenge set by Panama Papers."
Large nations including Iran, Egypt and Pakistan did not attend the summit, while China, Brazil, Indonesia and Saudi Arabia did not put their names to any of more than a dozen intitatives announced at the summit.
Russia only signed up to an intitiative promoting transaparency on taxes by large multinational firms.
"Corruption is at the heart of so many of the world's problems," the group said in a joint declaration. "We must overcome it if our efforts to end poverty, promote prosperity and defeat terrorism and extremism are to succeed."
"We commit to expose corruption wherever it is found, to pursue and punish those who perpetrate, facilitate or who are complicit in it, [and] support the communities who have suffered from it," they said.
Cameron promised Britain would take the lead by launching an international anti-corruption centre, sharing information on company ownership and forcing foreign property buyers to reveal the original source of their funds.
"I believe that corruption is the cancer at the heart of so many problems we face in the world," he said, highlighting problems including poverty, terrorism and money laundering.
Christine Lagarde, head of the Internatonal Monetary Fund, highlighted the "very strong link between corruption and lower growth."
"If you want growth, you have to be against corruption" Lagarde said, adding that she wants transparency and integrity to be a systematic part of IMF country surveillance."
US Secretary of State John Kerry said the summit could be "the beginning of something different."
"I've been shocked by the degree to which I find corruption pandemic in the world today," Kerry said, adding that it "destroys nation states."
"We have to say no safe harbour anywhere, and get the global community to come together," he said.
Colombian President Juan Manuel Santos highlighted the difficulties involved in waging a global fight against corruption, quoting an exchange in which World Bank President Jim Young Kim was told by the leader of an unnamed country: "What you call corruption, we call family values."
About a dozen protesters from anti-poverty group ONE gathered close to the summit, shouting slogans and holding a large placard reading "It's a scandal, 170 billion dollars stolen [annually]. Stop tax havens."
Several dozen protesters organized an Oxfam "offshore island” in London's Trafalgar Square, close to the summit venue.
In an embarrassment for Cameron ahead of the summit, broadcaster ITV showed video footage of him saying Nigeria and Afghanistan - two of Thursday's participants - were "possibly the two most corrupt countries in the world."
Speaking in parliament on Wednesday, Cameron appeared keen to mend fences, saying Nigeria and Afghanistan were "battling hard" against corruption and had taken "remarkable steps forward."
Reacting to his comments, Cobus de Swardt, head of anti-corruption group Transparency International, said the two nations were highly corrupt but their leaders had "sent strong signals that they want things to change."
"We should not forget that, by providing a safe haven for corrupt assets, the UK and its Overseas Territories and Crown Dependencies are a big part of the world's corruption problem," de Swardt said.
But officials from two offshore "tax havens" listed in the Panama Papers defended their record at the summit, saying global anti-corruption efforts should also focus on the role of major developed nations.
Nigerian President Muhammadu Buhari said the international community "has looked the other way for too long," by failing to recover assets stolen from developing nations and deposited with Western financial institutions.
Sitting on a summit panel with Cameron, Buhari cited the oil industry as an example of how corruption is a threat to Nigeria's economy and national security.