The summit takes place amid tight security and police warnings of "unprecedented" levels of protest.

It is the first time President Obama has visited Europe since taking office.

G20 LONDON SUMMIT World leaders will meet on Wednesday and Thursday 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 UK, the US and the EU.

He will begin a round of meetings with other world leaders, including the Russian President Dmitry Medvedev and the Chinese leader, Hu Jintao.

Whilst he was meeting with Mr Brown and his cabinet, Michelle Obama and Sarah Brown visited patients at a cancer care centre at Charing Cross Hospital.

International relations

The summit will provide a chance for the US president to demonstrate his new credentials, according to the BBC’s diplomatic correspondent James Robbins.

"Just not being George Bush is a huge advantage. George Bush was one of the most unpopular presidents of modern times," he said.

He also said that the UK and US were worried that their model of capitalism could be challenged by the French and Germans.

Detailed bargaining is expected to take place at these face-to-face meetings, although the two-day summit officially begins later when leaders from the G20, which includes the world’s most powerful economies, attend a dinner reception with the Queen at Buckingham Palace.

But expectations that the summit will come up with a definitive plan to stimulate the world economy are receding as rifts emerge between Europe and the US and UK.

Rifts

WEDNESDAY’S TIMETABLE 0800 Obama-Brown breakfast 1015 Obama-Brown press conference 1145 Obama-Medvedev meeting 1330 Brown begins further bilateral meetings 1400 Obama-Hu Jintao meeting 1430 Sarkozy-Merkel press conference 1730 Obama meets Queen at Buckingham Palace 1900 G20 reception with Queen at Buckingham Palace 2030 State dinner at Downing Street – beginning of summit

Earlier, President Sarkozy told French radio that although there were projects on the table, "as things stand at the moment, these projects do not suit France or Germany".

The French President is due to give a joint press conference with the German Chancellor Angela Merkel later.

French Finance Minister Christine Lagarde has already said that France would walk away from talks if its demands for stricter financial regulation were not met.

The G20 summit will be crucial for Gordon Brown, whose reputation for economic competence has been battered by the financial crisis.

He has sought to lead the drive for a global approach to the crisis and the meeting is an opportunity to restore his image as a competent economic manager.

But it is unlikely to be smooth sailing.

Jan Randolf, an analyst at Global Insight, said the meeting would be unlikely to live up to the hype surrounding it.

"The summit will almost certainly fall short of the lofty ambitions set out for it last year," Mr Randolf said.

Protests

The G20 talks will be accompanied by tight security, with police Cdr Simon O’Brien last week warning the capital was about to see an "almost unprecedented level of activity".

Workers in London’s financial district have been told to dress down or stay home to avoid provoking demonstrators.

Police leave during the two-day summit has been cancelled and six police forces will be involved in the £7.5m security plan.

The protests are expected to centre on the Bank of England in the heart of the City of London.

Some organisers have dubbed the event "Financial Fools Day" and say their protest will bring together anti-globalisation activists, environmental campaigners and others who are all demanding changes in the global economic system.

Campaigners have been handing out leaflets bearing slogans, such as "Hang A Banker" and "Storm The Banks".

Police said that although the majority of protests were expected to be peaceful, they were concerned about the actions of small groups.

City workers have been advised to swap business suits for casual clothing and police say people should work from home if they can.

Fearing damage to their property, some businesses in the financial district have already boarded up windows.

The prime minister said that police would act quickly if protests turned violent or if property was damaged.

The warnings follow a peaceful protest march of 35,000 people through London on Saturday, during which demonstrators demanded action on poverty, climate change and jobs.