JOHANNESBURG – The mastermind behind a rightwing extremist plot to kill former South African president Nelson Mandela and drive blacks out of the country was sentenced on Tuesday to 35 years in jail.

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 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.

All the accused were convicted of treason, but only five of murder and the plot to kill Nobel peace laureate Mandela.

Judge Eben Jordaan said Mandela would have been killed by a landmine planted by the Boeremag, causing chaos and bloodshed in the country, if he had travelled by road instead of 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.

The Boeremag – Afrikaans for "Boer Force", a reference to the descendants of the first Dutch colonisers – had planned to sow chaos in the country through bomb blasts.

One woman died and dozens of people were injured in blasts that shook the Johannesburg township of Soweto in October 2002.

The militia had also prepared five large car bombs to use in downtown Pretoria and Johannesburg, and were planning further bomb attacks when they were arrested, according to SAPA news agency.

Family members broke down in court after the sentences were announced.

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.

The government welcomed the sentences.

"These sentences will hopefully serve as a deterrent to those whose acts undermine or threaten state security," the justice ministry said in a statement.

Stephen Tuson, a law professor at the University of Witwatersrand said the sentences were expected given the gravity of the charges.

"Treason is an extremely serious offence. In the past it qualified for the death penalty and it ordinarily would call for the severest of penalties," he said.

First treason verdicts since apartheid

The trial lasted almost a decade and was one of the country's most expensive ever, costing the taxpayers over 30 million rand ($3.0 million, 2.0 million euros), according to local media.

The Boeremag members were convicted last August – the first guilty verdicts for treason since the end of apartheid in 1994.

Security was tight in the courtroom on Tuesday, as in 2006 two defendants escaped during a recess and were on the run for months, hiding on a farm, before being re-arrested.

Five years later the same pair, along with three others, escaped from the courtroom again, but were captured just minutes later.

If its plan had succeeded, the Boeremag intended to replace the government with white military rule and chase all blacks and Indians from the country.

Aside from former university lecturer Du Toit, 52, the group included a medical doctor, ex-soldiers and farmers.

The youngest of those sentenced is 32 and the oldest 74.

A father and three sons are among the convicted, local media said. Initially there were 23 defendants but one pleaded guilty earlier in the trial. He served four years before being released on parole.

Two others died during the trial.

Mandela, now 95, is critically ill but is being treated at his Johannesburg home after being discharged from hospital in September following a three-month stay.

The revered former statesman has faced several health scares in recent years due to lung problems that date back to his 27 years in jail under 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.