Paolo Di Canio
Sunderland say claims Paolo Di Canio has racist or fascist sympathies is insulting to both the "integrity of the club" and its new manager.
The Italian was appointed boss of the Premier League club on Sunday following the dismissal of Martin O'Neill, reports the BBC.
Di Canio, 44, apologised for causing offence with statements of a political nature but said he had been misquoted.
"It's been disappointing to read some of the reaction," said Sunderland chief executive Margaret Byrne.
Ex-foreign secretary David Miliband resigned from the board of Sunderland because of Di Canio's "past political statements" but Byrne said Di Canio's appointment had been turned into a "political circus".
"To accuse him now, as some have done, of being a racist or having fascist sympathies is insulting not only to him but to the integrity of this football club," added Byrne.
"Anyone who has met Paolo and spoken with him personally, as we did in depth before making this appointment, will know that he is an honest man, a man of principle and a driven, determined and passionate individual.
"It is disappointing that some people are trying to turn the appointment of a head coach into a political circus."
In a statement released by the club, Di Canio said he was hurt by his portrayal in the media since becoming Sunderland manager.
"Of course it hurts because people try to take your dignity and that is not fair," said Di Canio.
"I don't have a problem with anyone. I don't know why I have to keep repeating my story, to be defending myself on something that doesn't belong to me every time I change clubs."
Di Canio reportedly called fascist dictator Mussolini "a very principled ethical individual" who was "deeply misunderstood".
In 2005, he was pictured making a raised-arm salute to a group of supporters of Roman club Lazio, for whom he was a player.
"What I can say is that if someone is hurt, I am sorry," added Di Canio.
"Talk about racism? That is absolutely stupid, stupid and ridiculous.
"I expressed an opinion in an interview many years ago. Some pieces were taken for media convenience. They took my expression in a very, very negative way.
"But it was a long conversation and a long interview. It was not fair.
"When I was in England my best friends were [black England players] Trevor Sinclair and Chris Powell, the Charlton manager - they can tell you everything about my character.
"I don't want to talk about politics because it's not my area. We are not in the Houses of Parliament, we are in a football club. I want to talk about sport. I want to talk about football."
Di Canio first came to Britain as a player in 1996 when he joined Celtic, and followed his time in Glasgow with spells at Sheffield Wednesday, West Ham and Charlton.
He retired in 2008 after spells in Italy with Lazio and former Serie C1 side Cisco Roma, and was handed his first managerial role by Swindon in May 2011, replacing Paul Hart.