Boerestaat Party leader Robert van Tonder shot himself on Wednesday night and left a suicide note saying he could no longer stand the pain of the cancer he was suffering from, West Rand police said.
Spokesperson Inspector Patrys Greyling said Van Tonder is believed to have shot himself at 6.10pm on Wednesday.
"Although he has been ill for about two years, his death was a great shock for all of us," Van Tonder's widow Louise said on Thursday.
Greyling said his wife and daughter told people that he had been extremely weak and sickly.
Van Tonder had been suffering from cancer for the past two years.
He was at home on his Sandspruit smallholding in the Honeydew area near Randburg with his wife and daughter at the time of his death.
The Boerestaat Party said in a statement in Johannesburg:
"Robert, who was the bravest among the brave, is no longer with us. He devoted his life and his possessions to fighting for the survival of his people."
Van Tonder founded the Boerestaat Party in 1986 to fight for the restoration of the old Transvaal, Northern Natal and Free State Boer republics.
The party's leadership said only time would tell if Van Tonder's efforts would bear fruit.
"We believe his dreams and sacrifices were not in vain."
Fred Rundle, a prominent figure in rightwing circles and aclose friend of Van Tonder, said: "Robert was the intellectual among us. We mourn the death of a great Boer patriot."
The Herstigte Nasionale Party (HNP) said Van Tonder's death would leave a void in Afrikaner ranks.
He contributed much to maintaining of the Afrikaans language and never stopped striving for the freedom of the Afrikaner nation, HNP leader Jaap Marais said in Pretoria.
"He played a unique role that will not be filled easily."
Van Tonder broke away from the National Party in 1961 because of what he described as its betrayal of the old Boer republics.
He wrote a book, Boerestaat, in the 1980s which became a best-seller, Rundle said.
"This work outlined the cause of the Boer republics, and also received international recognition."
Rundle added: "Those who know, understand and support the cause of Boer nationalism can pay no finer tribute to Robert than by rededicating themselves to the cause of Boer independence."
Van Tonder often made headlines with controversial statements to further his cause.
In 1991 he said the Boer nation did not benefit much from an increase of tourists coming to South Africa.
Tourism promoted prostitution, venereal diseases, Aids, and a drop in moral standards, Van Tonder said.
He made several predictions of an uprising by those supporting the restoration of the old Boer republics if their demands were not heeded.
The people would embark on the kind of revolt that had plagued Northern Ireland for decades. He never gave details on what form of resistance he had in mind.
"If one plans something like that, one does not announce it," he said.
Van Tonder leaves his wife and six children. - Sapa