"Trump's Inaugural Address and his asking us to put America first sounds, to me, familiar to what was said in speeches going around Germany in the 1930's... and later Russia in the 1940's," the guitarist wrote.
"Pay attention people! Stand up for truth, compassion and togetherness. Don't settle for anything less. Reject lies, fear-mongering, misguided anger. The system wants us divided, so it's easier to control us emotionally."
Hammett also attacked Trump's campaign promise and the closing words of his inauguration address, "Make America Great Again."
"America never stopped being great! America is one of the wealthiest and most prosperous nations on the planet. I'm proud of America," Hammett tweeted. "To say America is not great, that it lacks greatness, sets up a scenario for manipulation and control from others - pay attention people!"
In his Twitter screed, Hammett also called climate change deniers "Earth killers" and said that the Alt-Right is "just another sneaky euphemism for white supremacy."
"If we don't put up a fight, we risk losing our rights," the guitarist said in conclusion. "Defend democracy from those who want to crush it."
In October, Metallica drummer Lars Ulrich said he would consider moving back to his native Denmark if Trump were elected president.
“I am a hundred percent Danish citizen. I pay taxes in the U.S.A., but I can’t vote in America,” he said. "Yes, certainly sometimes I think about moving home to Denmark… If Trump becomes President and everything goes to shit, I might make my way to the airport and ask if I can get back in again."