"I'm going to say one thing - f**k Trump!" he said, pumping his fists in the air.
"F-- Trump!" he roared, even though foul language and profanities are prohibited on public airwaves in the United States.
It was hardly the first time that De Niro, who was born in NY and lives in the city, attacked fellow New Yorker Trump. "It's no longer down with Trump, it's f- Trump!"
De Niro's comments on Monday echoed that call.
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.
A CBS spokesperson said in a statement to Deadline: "Mr. De Niro's comments were unscripted and unexpected".
As he took to the stage in NY to introduce a performance by Bruce Springsteen, the Oscar winning screen icon began a furious attack on the US President.
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With all of his many moves that many believe have divided America, and helped to encourage a risky social climate where guns are prized, President Trump was not a popular figure at the 2018 Tony Awards.
The gathered crowd seemed to agree with his message with applause, whoops and a standing ovation coming from the audience.
When asked about De Niro's remarks, Seawright said he probably would have chosen different words but seemed to praise De Niro for voicing people's "frustration".
The man behind Angels in America, legendary playwright Tony Kushner, spoke to reporters backstage about De Niro's spirited introduction for Springsteen, the AP reported.
Robert De Niro is my favorite rapper. "Who's gonna argue with him?"
In October 2016, De Niro made headlines by declaring that he wanted to punch Trump in the face.