Corruption is a bigger threat to the world than terrorist extremists, US secretary of state John Kerry said at a UK-hosted summit seeking to tackle global graft.
Mr Kerry praised the "courage" of David Cameron in calling the meeting, which has brought together 12 heads of state and government and a total of more than 40 countries in London.
The Prime Minister used his opening speech to warn foreign companies that own around 100,000 properties in England and Wales that they will be required to disclose their ownership.
It is one of a number of measures promised by the UK, with London seen as one of the prime international centres for people wishing to launder illicit assets.
He told delegates that the push - which he has made a theme of his premiership - was already bearing fruit, with more countries signing up to more tax transparency.
But he faced criticism from campaigners for failing to take stronger action against tax havens in British Overseas Territories and Crown Dependencies.
Former prime minister Gordon Brown is among those urging the premier to join other EU states in imposing sanctions against islands that continue to shield wealthy individuals' assets.
Speaking alongside Mr Cameron at the opening of the summit, Mr Kerry said he had been shocked at the extent of corruption in the world since taking on his role in the Obama administration.
"We are fighting a battle, all of us. Corruption, writ large, is as much of an enemy, because it destroys nation states, as some of the extremists we are fighting or the other challenges we face."
Anger would grow "unless we shut the door and show there is fairness in the system".
"There are sceptics as to whether this is a passing fancy ... or whether this is a beginning, a serious commitment," he said.
He warned that some states would seek to step in and fill the demand for secrecy if others were persuaded to open up - saying it was vital to show a zero-tolerance approach.
Mr Cameron called corruption "the cancer at the heart of so many of the problems we need to tackle in our world" - noting that illicit flows alone cost the world 1.26 trillion US dollars.
Tax-dodging destroys jobs, holds back growth, trap s people in poverty and can undermine security by making citizens more susceptible to the "poisonous ideology of extremists", he said.
The PM - who was embarrassed to be overhead calling Nigeria and Afghanistan "fantastically corrupt" in a conversation with the Queen days before the summit - stressed that it was "a challenge all countries need to address" including the UK and US.
He was sat on stage alongside Nigeria's president Muhammadu Buhari, who has made tackling corruption a central aim of his rule.
The PM is expected to meet with Mr Buhari and Afghan president Ashraf Ghani in the margins of the summit.
Mr Buhari appealed for the conference to agree measures that would see assets "stolen" from the country and hidden in Western capitals and other places to be returned.
Under the new rules, overseas firms will have to sign up to a new public register if they own or buy property or if they want to bid for central government contracts.
Amid a backlash over the Panama Papers, Mr Cameron last month announced that the overseas territories and Crown dependencies - such as the British Virgin Islands and Jersey - had agreed to provide UK tax and law enforcement agencies with full access to company ownership details.
But campaigners say the territories must be forced to adopt public registers, as the UK is and others are pledging to do at the conference.
Britain is also creating an international anti-corruption co-ordination centre in London.
Mr Ghani, who is attending the conference, said he had no issue with what the PM said about his country.
He told BBC Radio 4's Today: "He was describing the legacy of the past. Many actors, many factors combined to produce one of the most corrupt countries on Earth.
"The first part of addressing the problem begins with acknowledgement and we are partners in an effort to overcome this cancer."