At midday on Friday, Donald Trump will cap an extraordinary and improbable run for the White House when he takes the reins from US President Barack Obama.

"We're going to unify our country," Trump said in front of the Lincoln Memorial ahead of a fireworks display and following performances by Three Doors Down and Toby Keith. "We're going to do things that haven't been done for our country for many, many decades," he added. "It's going to change. I promise you."

The onetime beauty pageant sponsor has pledged to expedite deportations, begin building a wall on the border with Mexico and cancel billions of dollars in US funding to UN climate change programs. Trump, who beat out 15 of his fellow Republicans for the nomination, also intends to dismantle the Affordable Care Act, which gave more than 20 million people access to health insurance. He has also suggested potentially banning Muslims from entering the United States.

Trump will take the oath of office with 41 percent approval - the lowest of any modern US president. Three predecessors will attend, along with numerous dignitaries, including Trump's Democratic presidential rival: the former first lady and secretary of state Hillary Clinton, who won nearly 3 million more votes in the national tally after a bitter and divisive campaign.

'Very dangerous moment'

Alec Baldwin, Cher and Academy Award-winning documentarian Michael Moore joined hundreds outside a Trump property in Manhattan to demonstrate against the developer.

"A malignant narcissist and sociopath is in the Oval Office," said Moore, the director of "Capitalism: A Love Story" and "Fahrenheit 9/11." "This is a very dangerous moment."

City residents have vowed to break with Washington if necessary on health care, the environment and other issues during the Trump administration. New York Mayor Bill de Blasio was also on hand, pledging to defend the city and its residents from the harmful or racist policies many fear that Trump will attempt to impose.

Trump's other opponents have prepared for four years in the courts. The American Civil Liberties Union filed requests for details on conflicts of interest related to Trump's business affairs. The request on the eve of his inauguration marked the first salvo in what promises to be a contentious relationship between the ACLU and Trump.

"We have serious concerns that Donald Trump will be in violation of the Constitution on day one," ACLU executive director Anthony Romero told the news agency Reuters. "He has taken anemic steps to address the real questions around his businesses."

Trump intends to maintain ownership of his businesses but hand control to his oldest sons while president. He named his daughter's husband, Jared Kushner, to a senior advisory role in the White House, raising nepotism concerns. On Wednesday, Trump paid $25 million (23.4 million euros) to settle class action fraud suits against his eponymous university.

mkg/kl (Reuters, AFP, AP)