London - US President Barack Obama rejected Wednesday talk of deep divisions between the world's big economic powers gathering in London for a summit on the global economic crisis, emphasizing the human fallout from recession and the need for market reform.

The differences between world leaders attending the Group of 20 (G20) meeting of the leading industrialized nations and emerging economies had been "vastly overstated" the US president said at a joint press conference with G20 host and British Prime Minister Gordon Brown.

The leaders from the G20 nations, which also include France and Germany as well China and India were arriving in London for the summit as massive protests got under underway with the British capital's police mounting an unprecedented security alert.

Bankers in the city's financial heart have been advised to "dress down" for the day as landmark buildings, shops and hotels have been boarded up in preparation for the protest.

Commenting on reports of a rift between the US and Europe over Washington's calls for additional fiscal stimulus packages to help spur growth, Obama said governments around the world have launched their own plans for warding off the recession.

"I think there's broad recognition that in the midst of the greatest crisis we have seen since the '30s that governments are going to have to act, and certainly the US does not intend to act alone - and we are not," said Obama.

Acknowledging that 2009 was likely to be difficult year for people around the world, Obama said that people were concerned about "losing their jobs, losing their homes losing their savings and losing their pensions.

"We have responsibility to act with a sense of urgency," said Obama.

Both Obama and Brown also stressed the need for the G20 to move to beef up global market regulation to bring it into line with the fast-paced globalization.

Referring to the new tighter market regulatory regime unveiled in Washington last week Obama said his administration has proposed a set of "21st century rules of the road" for financial markets.

Speaking at the press conference, Brown said that he and Obama had agreed that the world leaders had to clean up the banking system so that an economic recovery could take hold.

The two leaders also emphasized their expectations that the London summit would be able to reach agreement on world market reform and stepping up international economic co-ordination to tackle the global recession.

Shrugging off reports that French President Nicolas Sarkozy had threatened to "walk out" of the summit if Franco-German demands on tighter regulation are not met at the meeting, Brown said he was sure that the French leader would still be there at the end of Wednesday's G20 working dinner.

"I will not associate myself with a summit that concludes with a statement of false compromises, which does not deal with the problems that confront us," Sarkozy said ahead of the summit.

The G20 summit is to open with a reception at Buckingham Palace and a working dinner in Downing Street, the office and residence of Brown.

Michelle Obama and Sarah Brown, the wife of the British leader, held a separate fireside chat in Downing Street as their husbands were holding political discussions early Wednesday.