The Met has warned that anarchists from the 1990 Poll Tax riots have been lured out of retirement by the prospect of violent clashes.
Tomorrow, four protest marches - known as the Four Horsemen of the Apocalypse - will converge on the Bank of England, under the banner of G20 Meltdown.
The anarchists intend to use two of these marches, the Black Horse and the Green Horse, as cover so they can get into the City. At last night's meeting, the group discussed whether to go in one block or in smaller swarms.
One ringleader, who did not reveal his name, said: "If we go there in a block, that will mean serious fisticuffs with the police."
A member of the Whitechapel Anarchist Group, who gave his name as Martin, said his group has been circulating leaflets urging violence. He said: "Are we going to storm the banks or just stand and wait until the police provoke us into a fight?"
One protester, from Greece, replied: "If we said we will storm the banks, then we should storm the banks."
The group, who said they want to "occupy the City for as long as possible", have organised a team who will "scout out the area" today to plan entrance and escape routes.
Protesters were also given a "legal briefing" on how to "get arrested properly" and how to secure police bail. Organisers recounted an earlier protest where one of their number hit a policeman on the head with a spade, crushing his vertebrae.
The Prime Minister today warned that police would take a zero-tolerance stance against threats to people and property. Mr Brown said: "No violence can be tolerated. No intimidation of people will be allowed. The police will act very quickly if there is any threat to people or property."