World leaders are gathering in London to discuss ways to resolve the worst financial crisis since the 1930s.
The G20 summit takes place amid tight security and police warnings of “unprecedented” levels of protest.
Workers in London’s financial district have been told to dress down and stay home to avoid provoking demonstrators.
President Barack Obama has arrived in London on his first visit to Europe since taking office, with hopes high that he can forge a new global deal.
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.
The two-day summit officially begins on Wednesday evening as the leaders from the G20, which groups the world’s most powerful economies, attend a reception with the Queen at Buckingham Palace.
But detailed bargaining will take place at a series of face-to-face meetings earlier in the day, including a breakfast meeting between Prime Minister Gordon Brown and President Obama.
The 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”.
Police leave during the two-day summit has been cancelled and six police forces will be involved in the Ã‚Â£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 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 of poverty, climate change and jobs.
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.
French Finance Minister Christine Lagarde has already said France would walk out of the summit if its demands for stricter rules are not met.
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.
“There may now be stronger agreement on financial regulation, but there is a long way to go before any consensus is reached on practical reform,” he added.