The leader of the far-right British National Party has used an appearance on BBC television to defend the Ku Klux Klan and to insist that his views on race had been misrepresented.
Outside the BBC Television centre in White City, London, about 500 people demonstrated against Nick Griffin's appearance on the Question Time programme last night.
Police are now questioning six people following the protests.
There were smaller protests outside other BBC premises including those in Belfast and Derry.
Around 30 demonstrators broke into the BBC's headquarters in London, while others clashed with police outside.
Six people were arrested and three police officers were injured, one being taken to hospital with a head injury.
The British Broadcasting Corporation defended its decision to invite Mr Griffin on the show, saying it was duty bound to be impartial.
The BNP had never appeared on the show before but was invited on after Mr Griffin and a colleague were elected to the European Parliament in June, with the party taking nearly 944,000 votes - a 6.2% share.
The BBC's invite sparked passionate debate in Britain and saw mainstream parties change tack and agree to share a platform with the BNP.
'We remain firmly of the view that it was appropriate to invite Nick Griffin onto the 'Question Time' panel in the context of the BBC meeting its obligation of due impartiality,' BBC deputy director general Mark Byford said.
Mr Griffin faced hostile questions and jeers in a show dominated by debates around BNP policy, with panellists from Britain's biggest three parties challenging the 50-year-old on quotes attributed to him.
While on the programme, Mr Griffin said: 'I've been relentlessly attacked and demonised over the last few days.
'I am not a Nazi. I never have been,' he said, adding: 'I do not have a conviction for Holocaust denial.'
He went on: 'Our country must remain fundamentally a British and Christian country... based on Western democratic values', saying he stood for people who felt 'shut out in our own country'.
Mr Griffin said beforehand that the furore around his appearance 'clearly gives us a whole new level of public recognition'.
The BNP website said it had been forced to take its normal pages offline due to a surge in hits.
The BNP wants to 'stop immigration and put British people first'.
Its membership is restricted to 'indigenous Caucasian' people, though that is set to change after a recent court battle.
Prime Minister Gordon Brown said the BNP leader going on the show was 'a good opportunity to expose what they are about'.
His appearance has dominated the British news agenda this week, raising issues of free speech and censorship, and is on the front page of most national newspapers today.
The Guardian said in its editorial it was 'questionable television'.
The Daily Mirror called it a 'propaganda coup for right-wing fanatics.'
The Daily Express said on its front page that Mr Griffin was 'a disgrace to humanity'.