Donald Trump pledged to put "America first" and vowed to "rebuild our country and restore its promise for all of our people" as he was sworn in as the 45th president of the United States.
The new president took the oath of office on the steps of the Capitol building as his family and outgoing president Barack Obama watched.
A Bible between them, Mr Trump was sworn in by Chief Justice John Roberts just before noon local time (5pm), before the two men shook hands.
Delivering his inaugural address, President Trump said: "Every decision on trade, on taxes, on immigration, on foreign affairs will be made to benefit American workers and American families.
"We must protect our borders from the ravages of other countries making our products, stealing our companies and destroying our jobs."
He added: "America will start winning again, winning like never before."
Promising to "make America great again", President Trump said: "From this day forward it is going to be only America first, America first."
President Trump's address broke with tradition as he sought to hammer home political points against globalisation and in favour of "American first" isolationism.
Imploring the country to come together, he said that a united America "is totally unstoppable".
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."
And as rain began to fall on the new president's shoulders, he 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.
President 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".
"We do not seek to impose our way of life on anyone but rather to let it shine as an example. We will shine for everyone to follow."
Earlier there were muted scenes as Mr 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 a bitter presidential campaign that divided America.
She appeared steely-eyed as she awaited Mr Trump's arrival ahead of the ceremony, but tweeted: "I'm here today to honor our democracy & its enduring values. I will never stop believing in our country & its future."
Mr Obama sent his own valedictory message to the American people, urging them to believe "not in my ability to bring about change, but in yours".
He tweeted: "It's been the honor of my life to serve you. You made me a better leader and a better man."
An estimated 800,000 people are thought to have travelled to Washington for President Trump's inauguration - not all of them supporters.
There were ugly scenes on the streets of the capital as protesters smashed windows, attacking buildings and clashing with both police and Trump supporters.
Similar protests took place around the world as President Trump was sworn in.
Despite suggesting he would start his work in earnest on Monday, President Trump wasted no time in settling himself into his new role.
He has already taken over Mr Obama's @POTUS Twitter handle, a stern-looking President Trump looking out amid a sea of American flags, but has not yet had the time to send his first Tweet from the new address.
More importantly, policies from the new administration have already appeared on the White House website.
Confirming the fears of many environmentalists, President Trump underlined his intentions on climate change, saying he is "committed to eliminating harmful and unnecessary policies such as the Climate Action Plan and the Waters of the US rule".
He also said the US would be "committed to a foreign policy focused on American interests and American national security", vowing to pursue "joint and coalition military operations" to defeat so-called Islamic State and other radical Islamic terror groups.
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