Donald Trump sworn in as 45th president of the United States (Watch Video)

Donald Trump has been sworn in as the 45th president of the United States.

Watched by dignitaries on the west front of the US Capitol in Washington, and a worldwide TV audience of millions, the uniquely divisive businessman-turned-politician took the oath of office administered by chief justice John Roberts on Friday at midday.

It was a moment of joy and vindication for Trump’s supporters and one of anxiety for those who oppose him. On a damp January day, the crowd on the National Mall appeared significantly smaller than for many past inaugurations. Some protesters clashed with police and hundreds of thousands are expected to march in the capital on Saturday.

“We, the citizens of America, are now joined in a great national effort to rebuild our country and restore its promise for all of our people,” Trump told the crowd as rain began to fall.

“Together we will determine the course of America and all of the world for many many years to come.”

The new president said: “Today we are not merely transferring power from one party to another ... but we are transferring power from Washington DC and giving it back to you, the people.”

He added: “January 20th, 2017, will be remembered as the day the people became the rulers once again. The forgotten men and women of our country will be forgotten no longer. Everyone is listening to you now.”

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Reviving some of the populist slogans from his election campaign, he told the crowd: “From this day forward, it’s going to be only America First, America First,” he said, and described “two simple rules”: “buy American and hire American.”

But in his 20-minute speech he also described a nation falling apart. “The crime, and the gangs, and the drugs that have stolen so many lives ... This American carnage stops right here and stops right now,” he said.

“Whether we are black or brown or white, we all bleed the same red blood of patriots,” he said. “Whether a child is born in the urban sprawl of Detroit or the windswept plains of Nebraska, they look up at the same night sky ... and they are infused with the same breath of life by the almighty creator.”

Turning briefly to foreign affairs, he attacked “radical Islamic terrorism” and said “we will eradicate completely [it] from the face of the Earth”.

Although Trump, who has no political experience, tore up the rulebook in his election campaign, on Friday he followed familiar solemn traditions. Trump and his wife, Melania, attended an hour-long prayer service at St John’s Episcopal church and then visited the White House. They were warmly greeted by Barack and Michelle Obama on a red carpet, and presented them with a gift box.

Obama and Trump shared a limousine ride to the US Capitol for a ceremony attended by former presidents Jimmy Carter, George W Bush and Bill Clinton, as well as Trump’s defeated election opponent, Hillary Clinton. Trump was sworn in using his own bible and one that belonged to Abraham Lincoln, ahead of his inaugural address, parade and balls.

The former reality TV host who rode a wave of populism to take on and beat the political establishment tweeted early on Friday: “THE MOVEMENT CONTINUES - THE WORK BEGINS!”

Later the official presidential Twitter account, @potus, was reset and all Obama’s tweets deleted. “@POTUS hasn’t tweeted yet,” the page said.

Obama, America’s first black president, could be seen departing the Oval Office for the last time. TV footage appeared to show him leaving a letter for his successor, even as a stepladder suggested that removals were already under way.