In an open letter to BNP supporters, he said his appearance on the flagship BBC political show was an unprecedented chance to present their views to the UK public.
Protests at Mr Griffin's presence on the Question Time panel grew today, with anti-fascist campaigners picketing BBC Television Centre and one MP arguing it was "irresponsible".
BBC director general Mark Thompson challenged the Government to ban the BNP from the airwaves if it felt Mr Griffin should not be allowed to take part.
But Prime Minister Gordon Brown said it was a matter for the corporation and he did not want to interfere with it.
Mr Griffin said in his letter, posted on the BNP's website: "Question Time is scheduled for 10.35pm tonight and will be a milestone in the indomitable march of the British National Party towards saving our country.
"Our violent opponents on the far left have promised to lay siege and barricade the studio venue, because they know only too well that this could be THE key moment that propels the BNP into the big time.
"Never before have we had the chance to present our patriotic, common sense solutions to Britain's nightmare situation to the public at large in such a prominent fashion."
The far-right politician claimed that Question Time would be "stage-managed" so the audience and panellists were "overtly hostile" towards him.
He said: "I will, no doubt, be interrupted, shouted down, slandered, put on the spot, and subjected to a scrutiny that would be a thousand times more intense than anything directed at other panellists.
"It will, in other words, be political blood sport. But I am relishing this opportunity."
For several days the BNP's website has had a prominent banner with a countdown to Question Time's broadcast.
An appeal by cabinet minister Peter Hain against Mr Griffin's appearance on the programme was thrown out yesterday by a special BBC Trust panel.
It ruled that it was "a question of editorial judgment" whether it was appropriate for the BNP to be represented on the flagship show and refused to block the move.
Mr Thompson today issued a strong defence of the decision to have the BNP on Question Time.
The BBC director general said the party's recent electoral success, which saw Mr Griffin elected as one of two BNP MEPs, meant it was right to invite him.
And he said only a legal ban, similar to that imposed on Sinn Fein in the 1980s, would lead the Corporation to consider breaching its "central principle of impartiality".
Mr Thompson wrote in the Guardian: "If there is a case for censorship, it should be debated and decided in Parliament. Political censorship cannot be outsourced to the BBC or anyone else."
Mr Brown said the appearance of Mr Griffin on the programme would present a good chance to expose his "unacceptable" views.
He told Real Radio in Yorkshire: "If on Question Time, they are asked about their racist and bigoted views that are damaging to good community relations, it will be a good opportunity to expose what they are about."
Asked whether Mr Griffin should have been invited on to the programme, Mr Brown said: "It was a decision of the BBC. We are not trying to interfere with the decision of the BBC."
The Prime Minister defended the decision to break with Labour's previous practice of refusing to share a platform with the BNP and to allow Justice Secretary Jack Straw to debate with Mr Griffin this evening.
"The issue is: should we have someone there? Jack Straw is a very experienced person who has had to deal with the BNP and their awful politics over a period of time," he said.
"I hope that the exposure of the BNP will make people see what they are really like."
Meanwhile, anti-fascist protesters leafleted BBC staff as they arrived for work today at Television Centre in White City, west London.
As security staff put up barriers for this evening's protest, Unite Against Fascism asked corporation workers to oppose the controversial debate.
The campaign group is expecting several thousand activists, including students from three London universities, to attend their demonstration.
Weyman Bennett, joint national secretary of Unite Against Fascism, said: "I don't believe the BNP are going to be taken apart in the debate.
"What they are going to get is a massive hustings for their fascist and racist politics and the price for that will be an increase in the number of racist attacks."
Labour MP Andy Slaughter, whose constituency which includes Television Centre, was outside the BBC to lend his support to the protesters.
He said: "I think it is just irresponsible. Cutting through the dinner party conversation about freedom of speech, the practical impact it is going to have on Muslim, black and Asian communities is reason enough not to give the BNP a platform."
Mr Hain also criticised the BBC's decision, telling GMTV: "Once you treat them as equal amongst the others they gain ground, we saw that in Nazi Germany.
"Griffin is saying this is the best thing that's ever happened to them and the BBC has given them an early Christmas present. It's disgusting.
"They've craved this kind of respectability all their political lives and at last the BBC has given it to them and I think the BBC should be criticised for that."
He added that it was not appropriate for him as a cabinet minister to take legal action against the corporation.
Former London Mayor Ken Livingstone, chairman of Unite Against Fascism, insisted the BBC should not give Mr Griffin air time unless the courts ordered it to do so.
"For the angry racist it's a trigger that turns into an attack," he told BBC Radio 4's Today programme.
"We first saw this when Enoch Powell made his Rivers of Blood speech. There was a huge surge of attacks on black conductors on our buses, and that is why I think you apply a different standard to the BNP to those parties that do not legitimise this sort of violence against minorities."
Mr Livingstone went on: "You all sit there in a comfortable world. But for the young Muslim walking home past a group of racist thugs who have been inflamed by what they hear the BNP say, he gets a kicking."
Also appearing on tonight's Question Time are Justice Secretary Jack Straw, shadow community cohesion minister Baroness Warsi, Lib Dem home affairs spokesman Chris Huhne and writer Bonnie Greer.
