Twenty white extremists have been jailed for between five and 35 years for plotting to kill Nelson Mandela in one of South Africa’s biggest post-apartheid treason trials.

The members of Afrikaner extremist group Boeremag, or white farmer force, were found guilty of a plot in the late 1990s and early 2000 to violently overthrow the democratically elected multi-racial government.

They also claimed responsibility for a bombing spree in Soweto that killed a woman in 2002.

Five of the sentences in the decade-long trial were suspended because of time already served.

The leader of the group Tom Vorster and four members of the group’s “bomb squad” were found guilty of trying to kill Mr Mandela by planting a roadside bomb on the way to a school visit.

The plot was only foiled when the anti-apartheid hero took a helicopter instead.

Judge Eben Jordaan said the group had been trying to destroy democracy in South Africa, just a few years after the end of the apartheid and the first multiracial election in 1994.

It was one of South Africa’s longest-running trials, costing 36 million rand (£2.3m).

The government welcomed the verdict.

“The decade-long trial shows how the attainment of freedom and democracy has changed the lives of the people in the country,” said spokesman Harold Maloka.

He added that the verdict should “act as a deterrent” against those threatening South Africa’s “hard-fought democracy.”