"It's no longer 'down with Trump, ' it's 'f**k Trump, '" De Niro said. Trump tweeted, adding that Trudeau was "Very dishonest & weak".
"Mr. De Niro's comments were unscripted and unexpected", CBS said in a statement.
De Niro, 74, wasted no time in hurling the F-bomb at the president as he introduced Bruce Springsteen's performance at the star-studded awards ceremony. While CBS was quick to catch the swear for US audiences, the word wasn't censored during the Australian broadcast. Canadian Foreign Minister Chrystia Freeland said her nation "does not conduct its diplomacy through ad hominem attacks".
De Niro, a noted liberal, has been increasingly critical of Trump, a Republican, at smaller showbusiness events, but none have the high profile and audience of the Tony Awards. The CBS television audience heard dead silence instead before he raised his arms - twice - and earned a sustained standing ovation.
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De Niro referred to Trump and his associates as a "gang of business-friendly thugs" and said that "business without humanity is not business, it's greed".
While he might have expected others in Hollywood to defend him, he probably didn't expect someone to suggest President Donald Trump himself was to blame after the comments.
"It's taken us about 50 years to realize how important diversification is and every political leader in Canada since that time has talked about the need for diversification", he says.
"Some Broadway guy saying 'Who's gonna argue with De Niro?' Ah, tens of millions, maybe". Kushner went even further, calling Trump's presidency "the Hitler mistake" that put a "borderline psychotic narcissist in the White House".