The "Boeremag" organisation had planned a right-wing coup in 2002 to overthrow the post-apartheid government.
The trial lasted almost a decade until the organisation's members were convicted in August last year -- the first guilty verdicts for treason since the end of apartheid in 1994.
"The accused had aimed to overthrow the government through unconstitutional methods that included violence," said High Court judge Eben Jordaan as he began the two-day sentencing hearing.
One woman died and dozens of people were injured in blasts that shook the Johannesburg township of Soweto in October 2002.
All 20 accused were convicted of treason, but only five of murder and the plot to kill Nobel peace laureate Mandela, South Africa's first black president.
The state is seeking life sentences for the group's leaders and bomb specialists, and 10 to 15 years in prison for the other defendants.
"We are hoping for a good conviction," said Paul Ramoloka, spokesman for specialist police unit the Hawks, who investigated the plot.
Security was tight around the courtroom, with police carrying out body searches of the public.
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