Donald Trump has been officially sworn in as the 45th president of the United States, ushering in a new political era.
The inauguration took place outside US Capitol in Washington, DC, on Friday, as protesters clashed with police nearby.
"Together, we will determine the course of America and the world for many, many years to come," Trump said in his inaugural speech.
"We will face challenges, we will confront hardships, but we will get the job done."
The Republican president added: "For too long, a small group in our nation's capital has reaped the rewards of government. Washington flourished, but the people did not share in its wealth."
He also pledged to unite the world against "radical Islam".
Taking aim at immigration and international trade, he said: "We must protect our borders from the ravages of other countries making our products, stealing our companies, and destroying our jobs. Protection will lead to great prosperity and strength."
After finishing his inaugural speech, Barack Obama, the outgoing president, told him: "Good job, good job".
Al Jazeera's Patty Culhane, reporting from Washington, DC, said: "Donald Trump focused on really saying 'everything is broken, but we're going to make it better'."
"It is shocking. I was here for both of Barack Obama's inaugurations and the crowd here is remarkably small compared to then. Another observation I have to share is that it is an incredibly white crowd."
About 28,000 security personnel, kilometres of fencing, street barricades, and trucks laden with sand were part of the security cordon around eight square kilometres of central Washington for the ceremony.
Thousands of protesters still converged on the capital. While the vast majority of the demonstrations passed peacefully, some protesters resorted to violence to express their dislike for the new president and his policies.
At least 217 people were arrested in a series of clashes, police said.
Police in riot gear used pepper spray and stun grenades to prevent the chaos from spilling into Trump's formal procession and evening balls.
Less than two kilometres from the inauguration ceremony, police gave chase to a group of about 100 protesters who smashed the windows downtown businesses including a Starbucks, a Bank of America and a McDonald's as they denounced capitalism and Trump.
The crowd, which carried at least one sign that read "Make Racists Afraid Again", also vandalised several cars and hurled rubbish bins and newspaper vending boxes into the streets before being dispersed by police.
Earlier, liberal activists with a separate group called Disrupt J20 intermittently blocked multiple security checkpoints leading to the largest public viewing area for the ceremony. Several were led away by police.
Disrupt J20 protest organiser Alli McCracken, 28, of Washington, said the group was voicing opposition to Trump's discriminatory comments about women, undocumented immigrants and Muslims.
"We have a lot of people of diverse backgrounds who are against US imperialism and we feel Trump will continue that legacy," McCracken said on a grey morning with light rain.
Trump supporters also flooded into the capital, many sporting shirts and hats bearing his "Make America Great Again" campaign slogan.
Other protests and demonstrations took place across the country, and in the rest of the world.
|Protesters block an entry point before the inauguration Trump in Washington, DC [Bryan Woolston/Reuters]|
Heading into the Oval Office shortly after the conclusion of his inaugural parade, Trump made good on a campaign promise to start dismantling his predecessor's healthcare law.
He signed an order on the Affordable Care Act that urged government departments to "waive, defer, grant exemptions from, or delay the implementation" of provisions that imposed fiscal burdens on states, companies or individuals.
Trump, a 70-year old businessman, has no previous political experience. He is a former reality TV star whose campaign dominated headlines with his shocking comments on minorities, women and rival politicians.
During the run-up to the November 8 election, he promised to ban Muslims from entering the US. He also promised to build a wall along the Mexican border to deter Mexicans from travelling to America. Later, a recording was leaked of him saying that he grabs women's genitals when the mood takes him. He called his main Democratic rival Hillary Clinton a "nasty woman" during a debate.
Speaking at the inauguration, Chuck Schumer, a senator from New York, said that America is now living in a "challenging and tumultuous time" in the run-up to the oaths.
"Faith in our government, our institutions and even our country can erode," he said. "Today, we celebrate one of democracy's core attributes: the peaceful transfer of power."
Chief Justice John Roberts administered the oath of office, which was witnessed by Trump's wife, the First Lady Melania Trump.
Mike Pence, vice president, was also sworn in front of a crowd of hundreds of thousands.