Ian Bone, founder of Class War, the now defunct group which first called for a party to mark Thatcher's death in 1994, said the parties would go on regardless of police tactics.
He said: "If they try to stop it, it will just take place as near as possible to Trafalgar Square.
"If they block one road, we will go up another. If they decide to close Trafalgar Square down, we will head up the Strand.
"They can block off whatever they like and it will happen in the streets round about."
He said he believed the disruption to the former Prime Minister's funeral would be minimal but was being blown out of proportion to provide an excuse to shut down the events.
He said: "I know of no one who is planning to hold any type of protest on Wednesday. I think they are trying to big that up in order to crack down on Saturday [tomorrow NZ time].
"No one knows how many are coming to Trafalgar Square, I think it has taken the authorities by surprise. I don't think they knew about it, I think the police thought it would be just 30 or 40 of the usual anarchist suspects. It is only later that they realised it would be much bigger."
Letters and emails have been sent to those the police believe to be organisers of the two events, calling on them to enter into a dialogue with officers, to facilitate their demonstrations. But Bone, who received a note despite his claim not to be directly involved in organising tomorrow's event, said: "The problem they have is that it is a party, not a demonstration or a march.
"So, it will just go on in the pubs if it cannot go on in the streets; that is what happened after the poll tax riot. It will be big and boisterous."
Asked about the possible deployment of rubber bullets and water cannon, a spokesman for the Metropolitan Police said: "We have no plans to utilise such equipment."
The fears of disruption follow similar parties hours after Thatcher died on Monday. Detractors gathered in Glasgow and Brixton to mark her passing.
- Independent