British Prime Minister David Cameron gathered world leaders in London Thursday to crack down on corruption — but critics say his mission has been undermined by Britain's tolerance for tax havens and his own indiscreet description of some attending nations as "fantastically corrupt."

The meeting has drawn a wide array of politicians from around the world, including U.S. Secretary of State John Kerry and the presidents of Afghanistan, Nigeria and Colombia. Banks, civil-society organizations and the International Monetary Fund will also be present at the gathering, which aims to produce a global declaration against corruption and break what Cameron has called the "taboo about tackling this issue head-on."

Cameron's own financial credentials were tarnished by last month's revelation — in leaked papers from Panamanian law firm Mossack Fonseca — that he had a stake in an offshore firm established by his late father. Cameron sold his shares in 2010, before he became prime minister.

And the British leader ruffled feathers before the summit when a television microphone caught him saying "leaders of some fantastically corrupt countries" were coming. Speaking at a Buckingham Palace reception with Queen Elizabeth II, he referred to Nigeria and Afghanistan as "possibly two of the most corrupt countries in the world."

Nigerian President Muhammadu Buhari and Afghan President Ashraf Ghani — elected in 2015 and 2014, respectively — have promised to curb corruption in their countries.

Buhari said he wasn't seeking an apology from Cameron, but wanted something "tangible" — the return of plundered Nigerian assets held in British banks.

"Corruption is a hydra-headed monster and a canker that undermines the fabric of all societies," Buhari said Wednesday at a pre-summit meeting. "It does not differentiate between developed and developing countries."

Cameron has said that battling bribery, money-laundering and other forms of financial wrongdoing is a priority for his government. But critics say London's financial district, the City, is awash with ill-gotten gains, and many of the world's leading tax havens are British dependencies or overseas territories, including Jersey and the British Virgin Islands.

In a move toward greater transparency, Britain has passed a law requiring British companies — including foreign firms that own British property — to disclose who really benefits from their ownership.

The British government said the register meant that "corrupt individuals and countries will no longer be able to move, launder and hide illicit funds through London's property market."

Britain said France, the Netherlands, Nigeria and Afghanistan were pledging to launch similar registers, and that more countries are due to follow suit.

Cameron has said that Britain's Crown dependencies and overseas territories have also agreed to share beneficial ownership information with U.K. law-enforcement bodies.

But charities and opposition politicians say Britain must go farther and insist that the territories' ownership registers are made public.

In a letter coordinated by aid group Oxfam ahead of the summit, some 300 economists argued that tax havens produce no economic benefit and "are distorting the working of the global economy." Signatories included American academic Jeffrey Sachs of Columbia University, France's Thomas Piketty and Nobel economics laureate Angus Deaton.

Writing in the Guardian newspaper, former British Prime Minister Gordon Brown said there must be automatic sharing of tax information and a "global blacklist" of tax havens "if we are to ensure no hiding places for tax evaders, no safe haven for tax avoiders and no treasure islands for the money launderers who hide an estimated $7.5 trillion of global wealth."

Cameron told lawmakers Wednesday that developed and developing countries alike had work to do on corruption, and "nobody is lecturing anybody."

"No country is perfect," Cameron said. "Nor, indeed, is any politician."