The biggest police operation for a decade will be launched next month for the G20 summit after protesters said they planned to bring chaos to London.
Business groups are warning staff not to schedule important meetings in the capital because of the likely disruption.
Tens of thousands of protesters are preparing to descend on the City and the Excel centre, where the two-day summit is being held.
Whitehall officials are also understood to be preparing contingency plans to move the conference elsewhere in the country if the security of the summit is jeopardised.
Although the summit is dubbed “the G20” there will be 22 countries represented at the meeting, including Argentina, Brazil, China, India, Indonesia, Russia, Saudi Arabia and the United States.
As well as the heads of state, more than 1,000 officials and civil servants are expected on April 1 and 2, including the heads of the International Monetary Fund and the World Bank.
Police are preparing to launch their biggest street security operation in Britain since the May Day riots nine years ago, with more than 5,000 Met and City of London police expected on the streets to maintain public order and protect the foreign leaders.
Sources said that it is a “unique” challenge.
One potential flashpoint is in the heart of the City, where thousands of climate change protesters are already planning to set up camp at the Carbon Trading Exchange.
An umbrella group called “G20 Meltdown”, representing 67 different protest groups from anti-war demonstrators to anarchists, promises to “reclaim the City, thrusting into the very belly of the beast: the Bank of England”.
The Daily Telegraph has learned of plans by four columns of protesters to march on the Bank of England from different train and Tube stations on the eve of the summit, April 1.
Local shops said police had already started to warn firms in the City and eastern parts of London of potential trouble in two weeks’ time.
Police will brief community groups and businesses next week on the latest intelligence of potential threats.
Part of the challenge for policing the operation is that many of the heads of state will be staying at their embassies in Belgravia.
This will mean that their motorcades will be travelling across London to where the summit is being held in a conference centre in the Docklands.
Sir Gus O’Donnell, the head of the civil service, said: " It is going to be challenging to move so many heads of state and government around London.
“Obviously we will do all we can to minimise disruption but we need to keep these various heads of states secure.”
Sir Gus also made reference to the particular risk around American presidents, after previous attempts on the lives of former heads of state.
He added: “American presidents, you know we know the history, so they are rightly very careful about security.”
The cost of the summit is already mounting. Yesterday the Cabinet Office said the cost to the taxpayer of the two-day event would be £19million.
This estimate includes all major costs – staffing, the venue, and event security and policing.
However, it does not include the disruption costs to businesses, which the London Chambers of Commerce said would be £5million a day. If there is trouble, this will rise “significantly”.
Colin Stanbridge, the chambers’ chief executive, said: "We are immensely proud that the G20 is coming to London but there is concern about the possible disruption to day-to-day business.
“We are confident that the Met and City police will be well prepared, but we would ask the authorities to consider the full implications for firms when making their security arrangements.”
The Federation of Small Businesses, which represents more than 200,000 small firms across the UK, has warned its members not to try to hold important meetings in London that week.
Stephen Alambritis, a spokesman, said: “We are warning our members not to arrange important meetings in London that week.”
A spokesman for the Cabinet Office said: "The Metropolitan Police is very experienced in policing events and demonstrations and deals with 4,500 a year.
“There will be a large security operation surrounding the G20 event, to deal with the event itself and any demonstrations that may occur.
“The Metropolitan Police are committed to, and will always facilitate, lawful protest.”
By Christopher Hope, Richard Edwards and Oliver Gregory