London - US President Barack Obama marks his debut on the world stage Wednesday, in a series of top-level meetings in London ahead of the G20 summit of leading economies.

As world leaders grapple with a deepening global recession, Obama also hopes to leverage his popular appeal to rebuild US relations around the world, which suffered under his predecessor George W Bush.

As well as London's G20 summit, this week includes a NATO summit hosted by France and Germany, US-EU talks in Prague and a state visit to Turkey for the new US president.

Obama's inauguration, during the worst economic downturn in decades, has given him little time to woo global leaders, as he has been plunged straight into tricky negotiations over ways to respond to the deepening crisis.

With his arrival in London Tuesday evening on his first big trip abroad, Obama sent a strong signal of the need for the United States and Europe to work together to tackle the world's challenges.

The US president's visit starts Wednesday over breakfast at Downing Street, with the British Prime Minister - and G20 host - Gordon Brown. The two leaders first met when the British Premier visited Washington a month ago.

Later in the morning, Obama is to meet Russian President Dmitry Medvedev, the latest in a series of small steps which the two powers have taken towards improving ties which had turned frosty in the latter period of the Bush administration.

Obama has already shown willingness to scale back plans for a missile-defence system in Poland and the Czech Republic. At this week's NATO summit, Russia can expect full relations to be restored, after they were suspended during the invasion of Georgia in August.

However, the US has refused to back down on NATO plans to expand eastward towards Russia's border.

Next on Wednesday's agenda, Obama is scheduled to meet with Chinese leaders, a week after he had rejected China's call to replace the dollar with a global reserve currency.

China is the United States' largest creditor, holding 740 billion dollars in US government bonds. Concerned about the value of those holdings, China's leadership has been critical of US monetary policies aimed at stimulating the economy.

On Wednesday afternoon, Obama and his wife Michelle are due at Buckingham Palace for an audience with Queen Elizabeth II, before the G20 leaders meet for a working dinner ahead of Thursday's summit.

During the course of the G20 summit, Obama is due to hold bilateral meetings with leaders of South Korea, India and Saudi Arabia.

From London, the US president travels on to France and Germany for the NATO conference, 60 years after the founding of the transatlantic alliance.

On Friday, Obama is separately due to meet French President Nicolas Sarkozy and German Chancellor Angela Merkel, for the first time since taking office.

Obama's whirlwind tour of Europe takes him onward to Prague for a US-European Union summit, before rounding his trip off in the Turkish capital Ankara, where he hopes to rebuild relations with Europe and the Muslim world that suffered under his predecessor.