Barack Obama joined Gordon Brown in a conspicuous display of unity today after kicking off a day of top-level negotiations before tomorrow’s G20 summit.

Mr Obama’s first foreign trip since his inauguration began with breakfast at Downing Street, where he and his wife were greeted by a rare round of applause from staff.

At the same time, a sour note was sounded by President Sarkozy of France, who warned in a French radio interview that he would not sign up to any “false comprises” at tomorrow’s summit in Docklands.

Mr Obama’s motorcade was just the first in a series of armoured convoys due to pass through the Downing Street gates today, although none other will compete in terms of size.

Mr Brown will also hold talks with President Medvedev of Russia, President Hu of China, the Indian Prime Minister, Manmohan Singh and Taro Aso, Prime Minister of Japan, all of whom will give their input to the summit communiqué.

But the two leaders who most threaten to upset Mr Brown’s plans for the G20 are not due at No 10 until this evening when all the G20 heads of state and government meet for dinner.

Mr Sarkozy and Angela Merkel, the German Chancellor, have already scuppered UK attempts to have the summit endorse a massive and coordinated fiscal stimulus and are now pushing for the summit to make clear progress on tighter regulation of financial markets to replace the “light-touch” Anglo-Saxon model blamed for the current crisis.

Asked on Europe 1 this morning about a Times report that he would be prepared to walk out of the summit if he does not agree with its conclusions, Mr Sarkozy replied: “I will not associate myself with a summit that would end with a communiqué made of false compromises that would not tackle the issues that concern us.”

He went on: “As of today, there is no firm agreement in place. The conversation is going forward, there are projects on the table. As things stand at the moment, these projects do not suit France or Germany.”

Mr Obama denied that there was any real disagreement on the need for governments to boost their economies, just normal discussions as to how best to do it.

“The truth is that that’s just arguing at the margins,” he said at a joint press conference with Mr Brown. “The core notion that government has to take some steps to deal with a contracting market place and to restore growth is not in dispute.”

Mr Brown also did his best to laugh off Mr Sarkozy’s walk-out threat. “I’m confident that President Sarkozy will be here not just for the first course of our dinner but will still be here when we complete our dinner this evening,” he said.

Hundreds of police have been drafted into London to reinforce security and thousands of protesters gathered outside the Bank of England for a “Meltdown Carnival” headlined by the left-wing rock singer Billy Bragg.

Mixing in with them were City workers who had deliberately “dressed down” – on police advice – to avoid being the targets of protesters.

More than 30 forces have provided specialist officers to the Metropolitan Police for the days around the summit when there will be unprecedented levels of protests and 40 diplomatic delegations requiring security.

While hopes of a new co-ordinated international stimulus of the world economy have faded, negotiations are proceeding on rules to curb excessive bonuses, control tax havens, extend regulation to hedge funds, introduce greater co-operation between national regulators and draw up fresh mechanisms to stop protectionism.

The summit at the ExCeL centre in Docklands comes as the World Bank said that the global economy would shrink by 1.7 per cent this year and the Organisation for Economic Co-operation and Development predicted a 2.7 per cent contraction, the worst since the Second World War.

But while Mr Brown pointed out that G20 governments had already agreed fiscal stimulus packages totalling more than $2 trillion, Mr Obama warned that the rest of the world could not simply rely on “voracious” American consumers to drive the global recovery.

“If there is to be new growth, it cannot just be the United States as the engine,” he said. “Everyone is going to have to pick up the pace.”

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