Donald Trump is now the President of the United States, the country's 45th.
He took the oath of office at 6am on Saturday (NZ time), bringing Barack Obama's eight years in charge to an end.
His Vice President, Mike Pence, took his own oath five minutes beforehand. The Mormon Tabernacle Choir performed between the pair's inaugurations.
Mr Obama and former Vice President Joe Biden were present for the transfer of power.
"It all begins today!" Mr Trump, a Republican, wrote in a note on Twitter earlier in the day. "I will see you at 11:00 A.M. for the swearing-in. THE MOVEMENT CONTINUES - THE WORK BEGINS!"
He stayed the night at an official guest house near the White House and began the day attending a prayer service at St John's Episcopal Church, a block away.
Mr Trump, wearing a dark suit and red tie, and his wife, Melania, clad in a classic-styled, powder blue ensemble, then headed into the White House for a meeting with outgoing Democratic President Barack Obama and his wife, Michelle.
Streets near the President's home were blocked to traffic by empty buses and dump trucks or temporary pedestrian security checkpoints. Checkpoints around the National Mall in front of the Capitol opened early to begin admitting guests. Some wore red caps bearing Mr Trump's "Make America Great Again" slogan.
Most of the area was orderly, but about 100 protesters shouted slogans near one checkpoint and linked arms to block people from entering. Police in riot gear pushed them back into an intersection to allow people attending the inauguration to reach the checkpoints.
Mr Trump, 70, takes office with work to do to bolster his image. During a testy transition period since his stunning November election win, the wealthy New York businessman and former reality TV star has repeatedly engaged in Twitter attacks against his critics, so much so that one fellow Republican, Senator John McCain, told CNN that Mr Trump seemed to want to "engage with every windmill that he can find".
An ABC News/Washington Post poll this week found only 40 percent of Americans viewed Mr Trump favorably, the lowest rating for an incoming president since Democrat Jimmy Carter in 1977, and the same percentage approved of how he has handled the transition.
His ascension to the White House, while welcomed by Republicans tired of Mr Obama's eight years in office, raises a host of questions for the United States.
Mr Trump campaigned on a pledge to take the country on a more isolationist, protectionist path and has vowed to impose a 35 percent tariff on goods on imports from US companies that went abroad.
His desire for warmer ties with Russian President Vladimir Putin and threats to cut funding for North Atlantic Treaty Organisation nations has allies from Britain to the Baltics worried that the traditional U.S. security umbrella will be diminished.
Reuters / Newshub.