Hundreds of thousands of people stood in the rain-splattered National Mall to see the 70-year-old Republican billionaire take the oath of office and deliver a stridently populist call-to-arms.
"From this day forward, a new vision will govern our land," Trump said, pronouncing an end to business as usual in Washington.
"Today we are not merely transferring power from one administration to another or from one party to another, but we are transferring power from Washington, DC. And giving it back to you, the people."
"From this day forward, it's going to be only America first."
"Together we will make America strong again. We will make America wealthy again. We will make America proud again. We will make America safe again. And, yes, together, we will make America great again," Trump said.
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Just moments before, the 70-year-old Republican billionaire placed his left hand on a bible used by Abraham Lincoln and recited the 35-word oath spoken since George Washington.
Trump's inauguration caps the improbable rise to power of the Manhattan real estate mogul, who had never before held elected office, served in the government or the armed forces.
A crowd of hundreds of thousands looked on, including outgoing president Barack Obama and Trump's defeated election opponent Hillary Clinton -- who narrowly missed out on becoming America's first female president.
Obama and his wife Michelle departed the Capitol by helicopter moments after the swearing-in ceremony, turning a page on eight years of Democratic leadership in the White House.
And as the incoming leader rallied his supporters for the swearing-in, throngs of his opponents also converged on Washington.
Most of the protests -- by an array of anti-racist, anti-war, feminist, LGBT, pro-immigration and marijuana legalization groups -- were noisy but peaceful, though sporadic violence marred the day.
Between 400 and 500 masked, black-clad protesters carrying anarchist flags smashed windows, lit fires and scuffled with riot police in downtown Washington, blocks from the parade held in Trump's honor, with over 90 people arrested for vandalism.
Even the peaceful protesters were intent on spoiling Trump's party -- letting out a deafening roar as the presidential limousine known as "The Beast" rolled by on the way to the White House.
"Not my president! Not my president!" they yelled, as the pro-Trump crowd in bleachers across the street chanted "USA! USA!"
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Take the power back
When Trump descended the escalators of his glitzy New York tower in June 2015, his run for office was dismissed and even mocked.
His staff, many shunned by friends for supporting a man who has been labeled a racist and bigot by his critics, will become power players in the White House.
In the primaries, Trump dominated a crowded Republican presidential field with smash-mouth rhetoric and star power. He rode that same wave of anti-elite sentiment to victory over Clinton in the November election.
"January 20th, 2017, will be remembered as the day the people became the rulers of this nation again," Trump said striking a populist and overtly political tone that is unusual for inaugural speeches.
"The forgotten men and women of our country will be forgotten no longer. Everyone is listening to you now."For Trump's critics, there was disbelief that a man who 19 months ago hosted "The Apprentice" is now leader of the free world.
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A short distance from the steps of the Capitol, where Trump was sworn in, police fired gas to disperse protesters. Stones were thrown and windows broken.
A 27-year-old financial worker from Tampa Bay in Florida, who did not want to give his name for fear of retaliation by his employer, said Trump's election victory had left him fearful.
"There is nothing to hope for except for grassroots efforts to oppose him," he said. A bigger anti-Trump rally is planned for Saturday.
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Time-honored traditions
At 70 years of age, Trump is the oldest man ever to begin work in the Oval Office.
He has vowed to tear up Obama's policies and re-examine decades-old alliances with Europe and in Asia.
"For many decades we've enriched foreign industry at the expense of American industry, subsidised the armies of other countries while allowing for the very sad depletion of our military," he said.
"We will reinforce old alliances and form new ones and unite the civilized world against radical Islamic terrorism, which we will eradicate completely from the face of the earth."
Earlier in the day, Trump had traced the steps of many presidents past, attending a prayer service at St. John's Church before heading to the White House, where he was greeted warmly by Obama.
"Mr President-elect, how are you?" Obama asked his successor, after having deposited a letter in the Resolute desk and left the Oval Office for the last time.
Later Friday Trump will enter that office. His team plans a rolling series of daily executive orders to roll back Obama's agenda.
But he arrives to the White House with a 37 percent approval rating, the lowest on record, according to a CBS News poll.
His inauguration was notably more sparsely attended than that of Obama in 2009 and 2013.
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