Robert De Niro said he was "depressed" by Donald Trump's election win and urged people to protest against his presidency "if things aren't being done right".
The Oscar-winning actor has been a vocal opponent of Mr Trump, saying he wanted to "punch him in the face" during the election campaign.
At the Los Angeles premiere of his new film, The Comedian, on Friday, Robert said: "I'm very depressed.
"We have to just wait and see how things go. We have to keep our eyes and (be) ever vigilant on the government."
Asked whether people should protest against Mr Trump, Robert replied: "Yes absolutely. If things aren't being done right, absolutely."
Anti-Trump protesters staged their third day of demonstrations across America on Friday following the Republican candidate's shock win over Hillary Clinton.
After the election result on Wednesday, Robert backtracked on his threat to punch the president-elect, telling US chat show host Jimmy Kimmel: "I can't do that now. I have to respect the position."
The Raging Bull star, 73, also compared his feelings about the election result to how he felt after the September 11 terror attacks.
In an interview with the Hollywood Reporter, he said: "I feel like I did after 9/11."
Robert, who plays an ageing comic in The Comedian, was joined at the premiere by his co-stars Leslie Mann, Danny De Vito and Danny's daughter Lucy.
Dame Helen Mirren also attended with her husband Taylor Hackford, who directed the film.
The Comedian, which had its premiere at Hollywood's Egyptian Theatre as part of AFI Fest, is released in US cinemas on January 13. A UK release date has not been confirmed.