US President Barack Obama stresses the "sense of urgency" needed to confront the global economic crisis, as protesters besiege the Bank of England.
The President met UK Prime Minister Gordon Brown at Downing Street ahead of the G20 summit of world leaders.
During his first visit to Europe since taking office, President Obama will hold a round of meetings with other world leaders, including the Russian President Dmitry Medvedev, and Chinese leader, Hu Jintao.
Thousands of anti-capitalist and climate change protesters are taking part in a series of demonstrations in London.
The key aim of the summit is to agree global action to do "all that it takes" to boost the world economy.
Police estimated that up to 4,000 protesters had gathered in the City of London. Police leave during the summit has been cancelled and six police forces are involved in the £7.5m security plan.
Environmental protesters staged a Climate Camp in the heart of the City, erecting tents in the road to block traffic.
Police have said they intend to use powers under the Public Order Act, rather than anti-terrorism laws, at the events.
Noisy protesters have converged on the Bank of England. Scotland Yard said 11 people had been arrested for being in possession of police uniforms.
A man claiming to be injured confronts police as they attempt to subdue the crowds near the Bank of England.
Some protesters got on to the roof of the Bank of England, while windows were broken at a nearby branch of RBS.
Groups under the G20 Meltdown banner marched from Tube stations to the Bank urging those who had lost their homes, jobs, savings or pensions to join them in following four "horsemen of the apocalypse".
The Queen hosts a reception for G20 leaders at Buckingham Palace ahead of a Downing Street dinner cooked by celebrity chef Jamie Oliver.
The G20 leaders dined on slow-roasted Welsh lamb accompanied by wild mushrooms, asparagus and Jersey potatoes.