WASHINGTON - President Donald Trump assumed power Friday with a fiercely nationalistic vow to put "America first," declaring a new political era after being sworn in as the 45th US head of state.
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, promising an end to business-as-usual in Washington
"From this moment on, it's going to be only America First."
"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."
"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.
Moments before, the incoming US leader had placed his left hand on a bible used by Abraham Lincoln and recited the 35-word oath spoken since George Washington.
While the US capital city no longer provides official crowd counts, the turnout was visibly smaller than for Barack Obama's two inaugurations, in 2009 and 2013, with sections of the Mall and bleachers along the parade route left empty.
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 legalisation 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 honour, 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!".
Trump's inauguration caps the improbable rise to power of the Manhattan real estate magnate who has never before held elected office, served in government or in the armed forces.
His speech was far from the typical optimistic inaugural address that tries to bridge political divides and lift Americans' gaze up to the horizon.
Trump painted parts of America as a dystopian hell, with mothers trapped in poverty and "rusted-out factories scattered like tombstones across the landscape." "This American carnage stops right here and stops right now," he said.
It was a deliberate and striking contrast from the uplifting message of Obama, the outgoing president who was among the dignitaries in attendance.
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.
At a Congressional luncheon afterward, Trump led a standing ovation for his defeated election rival Hillary Clinton, saying he was "honoured" that she and her husband, former president Bill Clinton, attended his inauguration.
When Trump descended the escalators of his glitzy New York tower in June 2015, his run for office was roundly dismissed and even mocked.
His message to his supporters on Friday was one of vindication: "The forgotten men and women of our country will be forgotten no longer. Everyone is listening to you now." 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.
For Trump's critics, there was disbelief that a man who 19 months ago hosted "The Apprentice" reality TV show is now leader of the free world - sworn in with a 37 per cent approval rating, the lowest on record, according to a CBS News poll.
Public interest lawyer Renee Steinhagen, 61, came from New York to join the protests.
"I'm doing this to express resistance to the change that await us," she said. "This administration seems more extreme than any other. This is a simple act of resistance. It's better than staying at home." .
Trump quickly got to work, signing a waiver for former general James Mattis to serve as defence secretary before a seven-year limit on former military personnel assuming the Pentagon's top post.
Mattis was confirmed by the Senate later Friday.
Trump also signed a proclamation for a "national day of patriotism." For the next few weeks his team plans a series of daily executive orders to roll back Obama's agenda.
Trump has also vowed to re-examine long-running alliances with Europe and in Asia.
"For many decades we've enriched foreign industry at the expense of American industry, subsidized 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 civilised world against radical Islamic terrorism, which we will eradicate completely from the face of the Earth."