Berlin (AFP) - A German court has sentenced an 87-year-old grandmother to 10 months in jail for Holocaust denial after a trial in which she insisted that Auschwitz was "not historically proven" to be a death camp.

That is "only a belief," said Ursula Haverbeck, dubbed Nazi-Oma (Nazi grandma) by the media on Friday.

Haverbeck is a notorious extremist who was once chairwoman of a far-right training centre shut down in 2008 for spreading Nazi propaganda.

She already has a criminal record with two fines and a suspended sentence for sedition.

Haverbeck was dragged back into court after she went on television in April to declare that "the Holocaust is the biggest and most sustainable lie in history".

Unapologetic for her comment, she had told the court cheerfully, "yes I said that indeed", according to media reports.

Haverbeck went as far as to challenge the court to prove that Auschwitz was a death camp, prompting ruling magistrate Bjoern Joensson to say "it is pointless holding a debate with someone who can't accept any facts".

"Neither do I have to prove to you that the world is round," he added.

Issuing his ruling Thursday, Joensson said: "It is deplorable that this woman, who is still so active given her age, uses her energy to spread such hair-raising nonsense.

"She is a lost cause," he added.

Nevertheless, the recalcitrant Holocaust denier is not without supporters.

At the trial in the northern city of Hamburg where anti-far-right activists had arrived in force to occupy most benches in the courtroom, Haverbeck's supporters were shouting outside: "Let us in."

She left the courtroom to applause.

"Of course" they won't accept this sentence, the Tageszeitung newspaper quoted her as saying.

Some 1.1 million people, most of them European Jews, perished between 1940 and 1945 in the Auschwitz-Birkenau camp before it was liberated by Soviet forces.