Los Angeles, Dec 14 (IANS): Actor, filmmaker and activist Rob Reiner feels that Donald Trump is "mentally unfit" to be the US President.

"Donald Trump is the single-most unqualified human being to ever assume the Presidency of the United States. He is mentally unfit," Reiner told variety.com.

"Not only does he not understand how government works, he has no interest in trying to find out how it works," he added.

Reiner, best known for his films like "A Few Good Men", "When Harry Met Sally…" and "The Princess Bride", is currently promoting "LBJ" -- a biopic on the life of former US President Lyndon Johnson.

"'LBJ' -- except for the Vietnam War, which is a terrible stain on his legacy -- was probably the most effective and successful President as far as domestic policy is concerned, second to maybe Franklin Roosevelt," Reiner said.

"When we are talking about Lyndon Johnson and Donald Trump, they couldn't be at further ends of the spectrum," he added.