Dr Toben was arrested on a plane at Heathrow Airport
The arrest and attempted extradition to Germany by British police of an alleged Holocaust denier would set a 'crazy and dangerous' precedent, say campaigners.
Dr Gerald Toben was arrested by British police under an EU arrest warrant issued by the German authorities.
That warrant accuses him of publishing material online "of an anti-semitic and/or revisionist nature".
Dr Toben has been remanded in custody after his extradition hearing on Friday was adjourned, but will face a bail hearing on Friday 10 October and a full hearing on 17 October.
Dr Toben, an Australian national, was convicted in Germany in 1999 for breaking a German law that prohibits denying or "playing down" the mass murder of the Jews under Hitler.
'No laws broken'
Officers from Scotland Yard's Extradition Unit arrested him on Wednesday while he waited on a plane at Heathrow airport.
Appearing before City of Westminster Magistrates' Court the same day, Dr Toben said he was the victim of "legal persecution".
He added: "It's a witch trial mentality in Germany concerning this matter, which is not the case in England yet."
Human rights campaigner James Panton, of the Manifesto Group, said that Dr Toben should not be extradited, because he had not broken any British laws.
"Extraditing this man - however unpleasant a character he may be - would set a crazy and dangerous precedent," he said.
"Toben has been arrested in the UK for being a Holocaust denier - but that is not a crime here.
"We are now entering a dangerous situation where you and I could potentially be arrested for crimes that other countries - but not the UK - deem to be offences."
The Crown Prosecution Service (CPS), whose lawyers are acting for the German authorities, argue that agreements signed in 2003 between the UK and other European countries mean that Britain is duty-bound to assist the German authorities.
A CPS spokesman said an extradition hearing would determine whether the requirements were met for surrender to the requesting territory under Part 1 of the Extradition Act 2003.
"The matters the judge are required to consider include whether the conduct constitutes an extradition offence."
Campaigners are also concerned that the UK is assisting the extradition of someone whose views appeared on the internet - rather than being published in Germany itself.
Nigel Farage, leader of the UK Independence Party, said that while his party in no way condoned Dr Toben's views, "not only has he not broken any UK laws, but in seeking to arrest him, Germany is claiming censorship rights to the entire internet network."
This view is shared by Australian free-speech campaigners who have accused Germany of trying to "legislate for the entire world" by treating downloadable internet material as a German publication.
Holocaust 'a lie'
Dr Toben, a German-born former schoolteacher, who now lives in Australia, has in the past described the Holocaust as "a lie".
His Australian-based website, the Adelaide Institute, carries the transcript of an interview in which Dr Toben says there is "no proof" that Hitler systematically exterminated the Jews, and that Auschwitz was a "transit camp".
Those who support Dr Toben's extradition say those views might qualify him for a British prosecution.
Dr David Cesarani, a lecturer in Jewish history and campaigner against British Holocaust denier Dr David Irving, said Dr Toben had a "long and obnoxious record" of anti-semitism.
"His views would probably qualify him for prosecution under laws against incitement to racial hatred in this country," he told the BBC's Jeremy Vine programme.
"While it may seem an unusual case we are part of a global judicial system and it is for the best."
Liberty director Shami Chakrabarti said: "When people are wanted in other democracies for actions that would be considered serious crimes in Britain, most would agree that extradition should be possible.
"But the danger with "fast-track" extradition is that there is less opportunity for justice to be seen to be done before someone is bundled off to another jurisdiction."