Judge Eben Jordaan convicted brothers Mike and André du Toit on the same charge last week.

He found that Du Plessis was a member of the Boeremag's inner circle and had taken part in planning a violent right-wing coup .

Du Plessis had attended numerous coup-planning meetings in 2001 and had travelled all over the country to recruit supporters.

He was the main speaker at many meetings, and usually warned that blacks would go "on the rampage" and kill whites once Nelson Mandela died, and that whites had to be ready to defend themselves.

At later meetings, Du Plessis openly talked about a coup and about taking over military bases. He said a Boer court would be formed after the coup to put "traitors", such as former president FW de Klerk and former military commander General Constand Viljoen, on trial.

Placing sharpshooters around airports and shooting down an airliner to create chaos were among the topics discussed at the meetings.

Judge Jordaan said Du Plessis had been involved in discussions about the formation of shadow commandos.

Jordaan's summary of the case against the 20 accused is expected to take another two weeks or longer to complete.