President Trump takes salute and warns world it's 'America First' now
Donald Trump pledged to make America great again after being sworn in as the 45th President of the United States.
In a populist eulogy, he vowed to unify a nation still bitterly divided by his election victory.
The new president took the Oath of Office on the steps of the Capitol building in Washington.
A Bible between them, Mr Trump was sworn in by Chief Justice John Roberts just before noon local time.
Addressing a watching world, he urged the country to come together, saying a united America "is totally unstoppable".
"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," he said.
"And, yes, together we will make America great again."
Mr Trump's 17-minute address broke with tradition, championing "American first" isolationism.
He vowed to rebuild the country "with American hands and American labour".
"We will follow two simple rules: Buy American and hire American," he said.
But the bitter division caused by his victory over Democrat candidate Hillary Clinton was evident on the streets of Washington and cities across America.
Washington police made "numerous" arrests after violent protests during the inauguration for rioting, the city's police department said.
From daybreak, hundreds of thousands of people had gathered at the National Mall to see the historic transfer of power.
Earlier there were muted scenes as Barack Obama left the White House for the last time before accompanying then President-elect Trump in a motorcade to the Capitol building, joining their wives, vice-presidents and former presidents for the swearing in ceremony.
Among them were former president Bill Clinton and his wife Hillary, who was defeated in the bitter campaign that split America.
Members of Congress were also in attendance, although more than 50 House Democrats had refused to attend the ceremony.
Ahead of the inauguration, watched by a global audience of millions, thousands of Trump supporters, many wearing hats declaring Make America Great Again, made the journey to the federal capital.
But legions of protesters also descended on the city, where a massive security operation was launched, including a heavily-guarded perimeter around the US Capitol building where Mr Trump delivered his inaugural address.
The new president said: "We are one nation, and their pain is our pain, their dreams are our dreams and their success will be our success.
"We share one heart, one home and one glorious destiny."
As rain began to fall, Mr Trump added: "America will start winning again, winning like never before.
"We will bring back our jobs, we will bring back our borders, we will bring back our wealth and we will bring back our dreams."
Mr Trump said the US would "seek friendship and goodwill with the nations of the world" but with the understanding that it is the right of all countries to "put their own interests first".
Mr Trump said the Oath of Office he had sworn was an oath of allegiance to all Americans.
"For many decades we have 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 added.
"We have defended other nations' borders while refusing to defend our own and spent trillions and trillions of dollars overseas while America's infrastructure has fallen into disrepair and decay.
"We have made other countries rich while the wealth, strength and confidence of our country has dissipated over the horizon.
"One by one, the factories shuttered and left our shores with not even a thought about the millions of American workers that were left behind.
"The wealth of our middle class has been ripped from their homes and then redistributed all across the world.
"But that is the past. And now we are looking only to the future.
Outlining his vision for the next four years, Mr Trump said: "From this day forward, a new vision will govern our land.
"From this day forward, it's going to be only America First, America First."