Mike du Toit, the ringleader of a white supremacist militia called Boeremag, was given the heaviest sentence of 35 years along with four other defendants over a botched 2002 plot to overthrow the post-apartheid government, the prosecution said.
The judge at the High Court in Pretoria sentenced the rest of the 20 militia members on trial to between 10 and 30 years depending on their degree of involvement in the plot, National Prosecution Authority spokesman Medupe Simasiku told AFP.
Judge Eben Jordaan said Mandela would have been killed by a landmine planted by the Boeremag, causing chaos and bloodshed in the country, were he not flown by helicopter to open a rural school in northern Limpopo province in 2002.
“They almost succeeded. It was extremely close,” head investigator Tollie Vreugdenburg told AFP.
However, several of the defendants will go home free men because the judge suspended 10 years of the sentences against them and took into account the time spent behind bars during the trial.
Family members broke down in court after the sentences were announced.
The trial lasted almost a decade until the Boeremag members were convicted in August last year — the first guilty verdicts for treason since the end of apartheid in 1994.
Security has been tight in the courtroom, as in 2006 two defendants escaped during a recess and were on the run for months, hiding on a farm, before being re-arrested.
All 20 accused were convicted of treason, but only five of murder and the plot to kill Nobel peace laureate Mandela.
A father and three sons are among the 20, local newspaper Pretoria News reported.
The Boeremag — Afrikaans for “Boer Force”, a reference to the descendants of the first Dutch colonisers — had planned to sow chaos through bomb blasts.
They then intended to take over military bases, replace the government with white military rule and chase all blacks and Indians from the country.
One woman died and dozens of people were injured in blasts that shook the Johannesburg township of Soweto in October 2002.
The group included former university lecturer Du Toit, 52, a medical doctor, ex-soldiers and farmers.
The youngest of those sentenced is 32 and oldest 74.
Mandela, now 95, is critically ill but being treated at his Johannesburg home after being discharged from hospital in September following a three-month stay.
The frail statesman has faced several health scares in recent years due to lung problems that date back to his 27 years in jail under the orders of the apartheid regime.
Mandela is admired for his lifelong sacrifice in fighting the racial segregation installed with apartheid in 1948.
He became South Africa’s first black president in 1994 after leading talks that ended white minority rule and put the African National Congress in power.