Donald J. Trump was sworn in as the 45th president of the United States on Friday — the beginning of a new era in American politics with the new president’s conservative platform set to reshape the nation.

Trump took the oath of office administered by Supreme Court Chief Justice John Roberts — and then addressed a crowd of hundreds of thousands.

The new president, with President Barack Obama standing by, struck a conciliatory note during his remarks, calling on Americans to unite behind his administration.

“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. Together we will determine the course of America and the world for many, many years to come. We will face challenges. We will confront hardships, but we will get the job done,” Trump said.

“Every four years, we gather on these steps to carry out the orderly and peaceful transfer of power, and we are grateful to President Obama and first lady Michelle Obama for their gracious aid throughout this transition. They have been magnificent. Thank you,” he said.

He then reiterated his vow to dramatically change the government and the direction of the country.

“Today’s ceremony, however, has very special meaning because today we are not merely transferring power from one administration to another or from one party to another, but we are transferring power from Washington, DC, and giving it back to you, the people,” he said to applause.

“For too long, a small group in our nation’s capital has reaped the rewards of government while the people have borne the cost. Washington flourished but the people did not share in its wealth. Politicians prospered but the jobs left and the factories closed,” Trump continued, blaming the political establishment for the malaise.

“Their triumphs have not been your triumphs and while they celebrated in our nation’s capital, there was little to celebrate for struggling families all across our land. That all changes starting right here and right now because this moment is your moment. It belongs to you.”

Trump praised his supporters and the “movement they created.

“You came by the tens of millions to become part of an historic movement, the likes of which the world has never seen before. At the center of this movement is a crucial conviction that a nation exists to serve its citizens. Americans want great schools for their children, safe neighborhoods for their families, and good jobs for themselves,” he said.

“For too many of our citizens, a different reality exists. Mothers and children trapped in poverty in our inner cities, rusted-out factories scattered like tombstones across the landscape of our nation.”

He decried rising crime in cities across America and repeated his call for a crackdown.

“The crime and the gangs and the drugs that have stolen too many lives and robbed our country of so much unrealized potential. This American carnage stops right here and stops right now. We are one nation and their pain is our pain. Their dreams are our dreams, and their success will be our success,” he vowed.

Trump also turned combative when touching on his key campaign themes — jobs, law and order, better trade deals with other countries and infrastructure improvements.

“I will fight for you with every breath in my body, and I will never, ever let you down. 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,” he thundered.

“We will build new roads and highways and bridges and airports and tunnels and railways all across our wonderful nation. We will get our people off of welfare and back to work rebuilding our country with American hands and American labor. We will follow two simple rules: Buy American. And hire American.”

‘I will fight for you with every breath in my body, and I will never, ever let you down.’

The billionaire businessman also said the US would vanquish ISIS.

“We will reinforce old alliances and form new ones and unite the civilized world against radical Islamic terrorism, which we will eradicate completely from the face of the Earth,” he said.

He finished his brief remarks by repeating his vow to “make America great again,” prompting Obama to remark, “Good job, good job” as the pair shook hands.

Rain began to fall as Trump delivered his inaugural address, which clocked in at just 16 minutes.

Afterward, he called out “Great job, Jackie!” to 16-year-old Jackie Evancho, following the “America’s Got Talent” star’s rendition of the national anthem.

During Trump’s speech, his administration took control of the White House Web site, wiping away Obama’s positions on environmental regulations, LGBT issues and civil rights.

In their place, the new White House put forth policies including “An America First Energy Plan,” “Making Our Military Strong Again” and “Standing Up For Our Law Enforcement Community.”

Vice President Mike Pence was sworn in first by Supreme Court Justice Clarence Thomas, followed by the Mormon Tabernacle Choir singing a rousing version of “America the Beautiful” accompanied by the US Marine Corps Band.

The ceremony was cordial, with Trump in a dark overcoat and his wife, Melania, clad in a powder blue cashmere jacket, standing at his side. Former President Bill Clinton and his wife, Hillary, were in attendance, as were former President George W. and Laura Bush and former President Jimmy Carter.

New York Cardinal Timothy Dolan offered a prayer before the swearing-in took place.

“God of our ancestors and lord of mercy, you have made all things. And in your providence have charged us to rule the creatures produced by you, to governing the world in holiness and righteousness, and to render judgment with integrity of heart,” Dolan said.

“Give us wisdom for we are your servants, weak and short-lived, lacking in comprehension of judgment and of laws.”

New York Sen. Chuck Schumer praised the peaceful transfer of power.

“We Americans have always been a forward-looking, problem-solving, optimistic, patriotic and decent people. Whatever our race, religion, sexual orientation, gender identity, whether we are immigrant or native-born, whether we live with disabilities or do not, in wealth or in poverty, we are all exceptional in our commonly held yet fierce devotion to our country,” he said, as many in the crowd booed and chanted, “We want Trump” and “Lock her up,” referring to Hillary Clinton.

“Today we celebrate one of democracy’s core attributes, the peaceful transfer of power. And every day we stand up for core democratic principles enshrined in the Constitution. The rule of law. Equal protection for all under law. The freedom of speech, press, religion. The things that make America America.”

Meanwhile, cops used pepper spray to quell violent downtown Washington protests during the inauguration.

With Bruce Golding