The Metropolitan Police have rolled out a security operation of "unprecedented" scale and complexity for the the G20 Summit this week. With the prospect of mass demonstrations around central London, they have told bankers to keep a low profile and warned that a hardcore of anarchists and anti-globalisation protesters are bent upon violence.

The series of demonstrations began yesterday with the Put People First march, organised by a coalition of 150 trade unions, charities and religious groups. However, there was little trouble as some 35,000 people marched from the Victoria Embankment to Hyde Park in a carnival atmosphere. One person was arrested, for being drunk and disorderly.

The flashpoints are more likely to begin on Wednesday 1 April, which campaigners have declared Financial Fools' Day. A demonstration will be held outside the Bank of England in which protesters plan to drink tea and sell home-baked cakes.

Camp for Climate Action, which organised protests at Heathrow airport and Kingsnorth power station, is planning a 24-hour camp in the City of London. Further protests are planned the next day in Docklands outside the ExCeL centre, where the summit will take place.

Smaller actions are also scheduled around the Square Mile, and police are monitoring social networking sites such as Facebook and Twitter to gather intelligence. On the day, officers will be watching the events on CCTV monitors at the police control centre in Lambeth, south London.

Commander Bob Broadhurst, of the Met, is leading the policing operation which he said would cost some £7.2m. The Met has cancelled all leave and placed its full force of 37,000 officers on standby. They will be joined by colleagues from a further five forces: the City of London Police, British Transport Police and the Essex, Sussex and Bedfordshire forces.

While police believe most demonstrations will be peaceful, they say some groups are inciting protesters to engage in violence. Material circulated online offers advice on how to attack the police, in one case suggesting throwing paint and a "constant hail of debris" to create "sterile zones into which the police won't go".

The police have advised bankers and lawyers to dress down to avoid being marked out as City workers, to leave their cars at home and use public transport, to cancel meetings and to forgo outdoor cigarette breaks.

Banks and multinational companies have engaged the services of security consultancies. Starbucks has hired security guards for its coffee shops and McDonald's has had staff trained by the Met in handling protesters.

"We consider it a high-risk situation," said Eden Mendel, of Kroll Security Consulting Group. "An unprecedented number of people are going to be in a small space at once. There is a risk things could get out of hand."

"Protests may surpass any since 1999 in intensity," said David Lea, of the international consultancy Control Risks, referring to the clashes between police and anti-globalisation campaigners outside that year's World Trade Organisation conference in Seattle.

Police admit that their security plans for the G20 Summit on Thursday and the protests the day before are likely to cause considerable disruption to Londoners throughout the week, with a spokesperson insisting: "We are trying to reduce the chaos it causes."

Peter Power, a former Scotland Yard security specialist who works for a consultancy, said: "The disruption is not just the event itself. It is almost certain that most of the disruption will be caused by the prevention."

The demonstrators have their own methods for dealing with outbreaks of violence. Mark Barrett, one of the organisers of the protest outside the Bank of England, said that if trouble kicked off stewards would order everyone to simply sit down, showing up the violent element as a minority.

Security clampdown

London has been put on alert for the week ahead. Here is some of the advice that has been given to people who might be affected by protests against the G20 summit:

Bankers Work from home; dress down; cancel meetings; use public transport.

Security staff "Anticipate anything"; liaise with police.

Canning Town residents Strongly advised to carry two forms of ID; prepare for stop and search; don't use cars.

Emergency Services Will be disrupted; ambulance stations relocated; more police patrols; no GP home visits.

ExCeL conference centre No access; local businesses including shops and nurseries to be closed.

Rail Three Docklands Light Railway stations will be shut on Thursday.

Roads Roads near the summit and in central London to be closed; traffic diverted.

Shops Protest-training for staff; extra security; police guards.

Police Rest days cancelled; 37,000 officers on standby.

Airports Leaders arriving at airports and RAF bases; extra police patrols.