Rudy Giuliani, under federal investigation over his dealings in Ukraine, has insisted he committed no crime while working as Donald Trump’s personal attorney.

“I committed no crime,” the former New York mayor told NBC, apparently unprompted during an interview about the forthcoming 20th anniversary of the 9/11 attacks, conducted at the 9/11 Memorial and Museum in downtown Manhattan.

“And if you think I did commit a crime, you’re probably really stupid because you don’t know who I am.”

Giuliani’s attempts to mine dirt on Joe Biden saw Trump impeached – and acquitted – for a first time.

Now under investigation himself, Giuliani has also seen his law licenses suspended in New York and Washington DC, for his part in propagating Trump’s lies about electoral fraud.

He protested to NBC New York that he could not be guilty of failing to register as an agent for a foreign power, a charge recently denied by another Trump ally, Tom Barrack.

“As the guy who put the mafia in jail, terrorists in jail, put [former mayor] Ed Koch’s commissioners in jail and the worst people on Wall Street, I’m not going to file [as an agent]?” Giuliani asked, referring to highlights of his time as a prosecutor before becoming mayor.

“I mean, that is crazy.”

He also said he was “more than willing to go to jail if they want to put me in jail. And if they do, they’re going to suffer the consequences in heaven, I’m not. I didn’t do anything wrong.”

Asked why he was willing to go to jail if he was innocent, Giuliani said: “Because they lie. And they cheat.”

He also claimed he was being treated unfairly compared to Andrew Cuomo, the New York governor who faces questions over his handling of nursing homes in the early stages of the Covid pandemic.

The FBI did not comment on Giuliani’s remarks.

NBC New York also asked if the deadly 6 January assault on the US Capitol, before which Giuliani spoke at a rally near the White House and demanded “trial by combat”, was a crime.

“I believe 6 January was a crime,” he said. “I believe [the rioters] committed the crime of trespass. I believe they did some destruction.”

Giuliani led New York City on 9/11 and in its aftermath and became known as “America’s Mayor”, a status he parlayed into a brief run for the Republican presidential nomination in 2008 and a lucrative consulting career.

He told NBC New York he was not concerned about tarnishing his legacy through his work for Trump, as he believed he would be vindicated.

In the bestselling book I Alone Can Fix It: Donald J Trump’s Catastrophic Final Year, the Washington Post reporters Carol Leonnig and Philip Rucker detail Giuliani’s attempts to help Trump overturn his election defeat, including just telling him, reportedly while drunk, to “just say we won”.

The authors also report that the former New Jersey governor Chris Christie, a longtime friend, asked Giuliani why he was energetically promoting Trump’s lies.

“Fuck legacy,” Giuliani is quoted as saying. “Legacy is what happens when you’re in the ground. I’m fighting for today.”