South African police said on Sunday they had arrested a Congolese man suspected of being the mastermind behind a rebel group's plot to overthrow the Kinshasa government.
Nineteen other suspected Congolese rebels have been arrested and were charged in a South African court on Thursday with plotting such a coup which prosecutors said they planned to finance through mineral concessions.
A 20th man was arrested in Cape Town on Friday.
"He is the mastermind. We have been observing these guys. We have a list of individuals and their names," said a spokesman for the Hawks, the police unit which had tracked the group.
The suspect will be flown on Monday or Tuesday to court in Pretoria, where the other 19 appeared.
The police said they received information in September that the group, a wing of a little-known rebel militia called the Union of Nationalists for Renewal (UNR), were planning a coup in the Democratic Republic of Congo.
The group required large quantities of arms and military training, the cost of which they would pay back, if they were successful, with mining concessions in the resource-rich country.
Court documents say the UNR consists of 9,000 rebels opposed to the rule of President Joseph Kabila and want him removed by "unconstitutional" means.
Prosecutors have played down comments by law enforcement officials that those arrested had links to M23 rebels, who have carved out a fiefdom in Democratic Republic of Congo's North Kivu province.