Met Police Commissioner Sir Ian Blair has defended the decision to invite a controversial Muslim scholar to speak at a police-sponsored conference.
Sir Ian Blair said Mr Ramadan's would engage young Muslims
Tariq Ramadan's US visa was revoked in 2004 for "security reasons".
Mr Ramadan has been accused of supporting attacks in Israel and Iraq. He publicly condemned the 11 September and London attacks.
Sir Ian said his was an important voice that would be listened to by young radicalised Muslims.
He said Mr Ramadan, who was also previously banned in France, had been visiting the UK for five years.
"We can't understand why he's banned in the US, (and) he is no longer banned in France," Sir Ian said.
Mr Ramadan is speaking at the Middle Path conference on 24 July, an event partly funded by the Association of Chief Police Officers (Acpo) and the Metropolitan Police.
His invitation was criticised by Norman Brennan, a serving police officer who heads the Victims of Crime Trust.
He said: "The police and Home Office should have nothing to do with this. To allow him in the UK to pontificate with his poison is bad enough, but to also sponsor him is just ridiculous.
"Sometimes Acpo and police organisations try to be so PC that they cross the line and in my view they have crossed the line. They should withdraw their support."
Sir Ian said: "Clearly this man has views about the struggle in Palestine and the struggles in Iraq which I find very difficult or offensive.
"(But) unless we hear these voices we are going to be in trouble.
"He has been coming here for years and if his voice says, which is what I want to hear him saying, that what happened here in London was totally outrageous and un-Islamic then that is a very useful voice for young people in Britain to hear."
Mr Ramadan called on Muslims to condemn the attacks on London "with the strongest energy".
He said: "Criminals, no doubt, will continue to kill, but we shall be able to respond to them by demonstrating that our experience of human brotherhood and mutual respect is stronger than their message of hate."