Gordon Brown invokes Martin Luther King's "fierce urgency of now" speech
Gordon Brown has called for banks and financial markets to adopt "family values" as world leaders gather in London for Thursday's G20 summit.
US President Barack Obama arrived in the UK on Tuesday evening ahead of what has been billed as a potential turning point in tackling recession.
The UK and US leaders talked during the president's flight, officials said.
Earlier, addressing religious leaders, Mr Brown said markets must abide by common values such as fairness.
Mr Brown, who met with the leaders of Indonesia and South Korea on Tuesday, has stressed the main aim of the summit - taking place in London's Docklands - must be to "clean up" the global banking system.
Mr Brown spoke to the president during his flight to "take stock" of their positions ahead of the summit and to identify "outstanding issues" which still needed work on.
But officials are still arguing over the final wording of the communique the leaders will deliver at the end of the meeting amid signs of differences on certain issues.
The Times says French President Nicolas Sarkozy has threatened to walk out if he fails in his demands for a worldwide financial regulator - an idea opposed by Britain and the US.
Instead, countries are expected to agree greater cooperation among national regulators.
However, ahead of the summit there has been progress on tougher regulation of tax havens and Mr Brown is holding a series of talks with world leaders arriving for the summit as efforts continue to agree concerted action.
Police search derelict buildings, drains and a harbour area at the summit venue
In an address to religious leaders gathered at St Paul's Cathedral about the aims of the G20, Mr Brown said that "markets need morals".
During his passage about families' values he said people did not encourage their children "to seek short-term gratification at the expense of long-term success".
"And our task today is to bring financial markets into proper alignment with the values held by families and business people across the country."
There was also a positive outlook for the summit given by President Obama in an interview with the Financial Times: "With respect to the stimulus, there is going to be an accord that G20 countries will do what is necessary to promote trade and growth.
"The most important task for us all is to deliver a strong message of unity in the face of crisis."
Shadow chancellor George Osborne, for the Conservatives, said the UK government's entire economic strategy had fallen apart when Bank of England Governor Mervyn King said that "this country could not afford to borrow more for a second fiscal stimulus".
Colette McBeth reports from the City of London ahead of the protests
But he said there was "common ground" on the G20 and welcomed the summit being held in London.
On financial regulation, he told MPs the test would be whether the G20 "engages with the difficult issues or ducks them".
But he said the UK government would have "more credibility" on bankers' bonuses and offshore tax havens if the prime minister had sacked Lord Myners, the City minister accused of signing off RBS bank boss Sir Fred Goodwin's £703,000 pension.
The Trades Union Congress said the summit must make a "real difference" to people in the UK and that failure to agree a package to support growth and jobs could not be contemplated.
The G20 has prompted one of the biggest security operations ever mounted in the UK, with mass protests planned by anti-capitalist groups and the Stop the War Coalition among others.
Mr Brown said no violence or intimidation would be tolerated from protesters and that police would act "very quickly" to protect people or property under threat.
Mark Barrett, from the anarchist protest group G20 Meltdown, told the BBC its demonstrations, in the City of London on Wednesday and outside the summit on Thursday, would be peaceful.
G20 LONDON SUMMIT
World leaders will meet later this week in London to discuss measures to tackle the downturn. See
our in-depth guide
to the G20 summit.
The G20 countries are Argentina, Australia, Brazil, Canada, China, France, Germany, India, Indonesia, Italy, Japan, Mexico, Russia, Saudi Arabia, South Africa, South Korea, Turkey, the United Kingdom, the US and the EU.
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