British National Party leader Nick Griffin was unrepentant after being filmed by the BBC attacking Islam as a "vicious wicked faith".
In an interview, he refused to say sorry and said the "Islamification" of the West had partly happened by rape.
But he did apologise for comments made by other BNP activists shown on BBC documentary The Secret Agent, broadcast on Thursday, confessing to race crimes. Three of them have been expelled from the party, Mr Griffin said.
Reporter Jason Gwynne spent six months infiltrating the BNP's West Yorkshire branch with the help of a former local organiser.
In the documentary, footage recorded at a meeting in Keighley shows Mr Griffin warning the audience to "stand up" to Muslims.
He said Islam "has expanded through a handful of cranky lunatics" and "is now sweeping country after country".
Speaking to BBC's Newsnight, he refused to apologise for his comments and continued to attack Islam.
Asked whether he thought Islam had expanded due to rape, a theory he had previously stated, he said: "It's one of the ways in which it's expanded, it's also expanded as the Koran tells its followers to do so - it's expanded at the point of the sword."
He added: "You give me 20 minutes or an hour - a special programme to dissect the Koran and I will show you that we have a monster in our midst."
Mr Griffin accused the BBC of selective editing in the documentary and said his full speech had discouraged attacks on communities.
West Yorkshire Police said a number of issues raised in the programme would be investigated and they would review tapes on Friday.
"Working with our colleagues in the Crown Prosecution Service, we will be reviewing the material to identify what, if any, information of evidential value it contains and decide on the appropriate action," police said.
In the film, one BNP member told Mr Gwynne how he kicked and punched a man during the 2001 Bradford riots.
Another member said he wanted to "blow up" Bradford's mosques with a rocket launcher.
And BNP council candidate Dave Midgley is shown saying he squirted dog faeces through the letterbox of an Asian takeaway.
Mr Griffin told Newsnight he was appalled by the comments and said: "There's no defence at all for what those people said."
Three party members have been expelled and a fourth is facing an internal disciplinary tribunal, he said.
But he accused Mr Gwynne of provoking the men into saying these things - a claim denied by the programme makers.
The Muslim Council of Britain (MCB) commended the BBC investigation.
Iqbal Sacranie, Secretary-General of the MCB, said: "The BNP have long been exploiting a loophole in our current legislation which outlaws incitement to racial hatred but does not forbid incitement to religious hatred.
"This documentary provides additional evidence of the immense harm this is doing to community relations in our country."
Earlier this month, Home Secretary David Blunkett unveiled plans to make inciting religious hatred a criminal offence.
In parliament the BNP was dubbed a party of "vile Nazis and thugs" by Commons Leader Peter Hain, amid condemnation from all parties.