"I watched him. and truly believe he may be "punch-drunk"," he said.
The crowd at New York's Radio City Music Hall cheered his remarks, though he was later criticized for stooping to Trump's name-calling level.
If you thought Donald Trump was going to let Robert De Niro get away with his f-bomb at the Tony's, you are mistaken. The expletives sparked a roaring reaction from the audience, with numerous celebrities standing up.
"Mr. De Niro's comments were unscripted and unexpected".
De Niro, a noted liberal, has been increasingly critical of Trump, a Republican, at smaller show business events in the past year, but none of them had the profile enjoyed by the Tony Awards.
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"It's no longer 'down with Trump, ' it's 'f**k Trump!'" De Niro said.
De Niro has been an outspoken critic of President Donald Trump, and played the special counsel Robert Mueller on "Saturday Night Live". Of course, this user is referring to former FBI Director Robert Mueller and the Special Counsel investigation on Trump's 2016 presidential campaign and any Russian interference.
After De Niro's comments went viral Sunday night, CBS quickly responded as well.
Hours later, Trump addressed the actor in a tweet. On Monday, the actor apologized to Canadians for the "idiotic behavior of my president".
"It's a disgrace. And I apologize to Justin Trudeau and the other people at the G7".