BNP leader Nick Griffin has sparked outrage with a series of tweets branding Nelson Mandela a "murdering old terrorist".
The far-right politician also mocked the 94-year-old former South African president's lung condition.
He wrote: "Saint #nelsonmandela on last legs it seems. Make sure to avoid BBC when the murdering old terrorist croaks. It'll be nauseating.
"'Statesmen' must be judged on results not rhetoric. Before Mandela, South Africa was safe economic powerhouse. Now crime ridden basket case.
"No surprise #Mandela's lungs are shot - all those burning tyres. Smoking necklaces very bad for the health."
Mandela's family gathered at his bedside on Saturday night after he was taken to hospital with a serious lung infection.
He is suffering from a recurrence of the condition which has put him in hospital three times since December .
Mandela served 27 years of a life sentence in prisons including the infamous Robben Island for sabotage and conspiracy against the oppressive South African Apartheid regime.
He was released in 1990 and was elected president of South Africa four years later in the country's first free and fair elections.
Many people took to Twitter to criticise Griffin's comments.
Peter Hain, the South Africa-raised former Labour minister and hardline anti-Apartheid campaigner, tweeted that it was a "nasty vicious attack".
Ruth Rosenau, a Labour councillor in Stoke-on-Trent, tweeted: "#Mandela - a towering figure in world history & an inspiration to millions. Nick Griffin - what will you be remembered for?"
Bob Woods, who tweeted from BrummyTaff, said: "#NickGriffin Insults #Mandela & shows what a small minded odious irrelevant little man he is. Not fit to tie his laces."