Barack Obama vowed to speak up if Donald Trump threatens core US values and reassured Americans "we'll be okay" Wednesday, in a political swansong after eight years as president.

During his final press conference and public appearance before Trump's inauguration on Friday, the 55-year-old said he was stepping back but would return to the political breach in extremis.

"I want to do some writing, I want to be quiet a little bit and not hear myself talk so darn much. I want to spend precious time with my girls," he said.

But, he added, any effort to enforce systematic discrimination, erode voting rights, muzzle the press or round up young immigrants, would cause him to speak out.

"There's a difference between that normal functioning of politics and certain issues or certain moments where I think our core values may be at stake."

During the campaign Trump vowed to ban Muslims from entering the United States and deport millions of illegal immigrants, many of them Latin Americans long-settled in the country.

His strident tone since winning election and the contrast with eight years of Obama's liberal agenda has given the country something akin to political whiplash.

Trump's supporters are euphoric that political business as usual may be over, while his detractors are fearful of a mercurial and untested leader.