G20 protesters are circulating detailed pamphlets advising people on how to win street battles against riot police and what to do if arrested.
Thousands of people are expected to bring the City of London to a standstill on Wednesday and Thursday, as popular anger over government bailouts of the banking sector reaches fever pitch.
The vast majority of protests are likely to be peaceful but the Metropolitan Police claims extremist and anarchist groups might resort to violence.
The online pamphlets suggest certain groups are advising their followers on how to beat the police should things turn rough. One document, called "Guide to Public Order Situations", explains how to breach lines of riot police using a "snow plough" human formation; throw rape alarms to make it hard for the police to give orders; resist baton and horse charges using nets; and "de-arrest" seized protesters.
Speaking after the Put People First march in Hyde Park, London, on Saturday, an unidentified member of an anarchy group told a crowd: "We want to see resistance on the street on Wednesday. If the police are ready for you, go and fight them. If they're not, give them a surprise."