The inauguration took place outside US Capitol in Washington, DC on Friday, ushering in a new political era.
“We will face challenges, we will confront hardships, but we will get the job done.”
He also pledged to unite the world against “radical Islam”.
Trump, a 70-year old businessman, has no previous political experience.
During the run up to the November 8 election, he promised to ban Muslims from entering the US. He also called for a wall to deter Mexicans from travelling to America.
Speaking at the inauguration, Chuck Shumer, a senator from New York, said that America is now living in a “challenging and tumultuous time” ahead of 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.
Clashes and protests
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.
Black-clad activists protesting Trump’s inauguration smashed shop and car windows in the capital on Friday and fought with police in riot gear who responded with pepper spray and stun grenades.
About 500 people, some wearing masks and kerchiefs over their faces, marched through the city’s downtown, breaking the windows of a Bank of America branch, a McDonald’s outlet and a Starbucks shop, all symbols of the American capitalist system.
“The message I want to send is that Trump does not represent this country.
Not far from the White House, protesters scuffled with police, at one point throwing aluminum chairs at an outdoor cafe.
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 gray morning with light rain.
Other protests and demonstrations took place across the country, and in the rest of the world.