The security operation for the day is likely to cost around €93.8m
Donald Trump has pledged a "great national effort" to rebuild America after being sworn in as the 45th President of the United States.
The 70-year-old billionaire took the oath of office on the west steps of the US Capitol in Washington and brought the curtain down on the presidency of Barack Obama.
In his inaugural address, Mr Trump promised to transfer power from Washington to the people, and pledged a "great national effort" to rebuild the country.
Mr Trump said January 20th would be remembered as "the day the people became the rulers of this nation again".
"The forgotten men and women of our country will be forgotten no longer," the new President said.
Mr Trump added: "From this day forward it's going to be only America first.
"We must protect our border from the ravages of other countries. I will fight for you with every breath of my body and I will never let you down."
While light on policy details, he pledged to 'eradicate' Islamic extremist terrorism "from the face of the Earth".
He stressed that every "every decision on trade, on taxes, on immigration, on foreign affairs will be made to benefit American workers and American families”.
Hundreds of thousands of people have packed onto the National Mall to witness one of the most remarkable transfers of power in American history.
Thousands of protesters have also poured into Washington.
The new Republican President is expected to begin issuing executive orders within hours, according to aides, keeping promises he made on the campaign trail.
Mr Trump swept to victory in November's election after an unconventional campaign and a message of shaking up the political establishment in Washington.
It remains unclear exactly what steps he will take to deal with immigration, healthcare reform and job creation.
Some of his statements have caused alarm amongst international friends and foes.
The world also waits to see if he will continue his addiction to Twitter.
As they welcomed Mr Trump as their new commander-in-chief, Americans also said farewell to Mr Obama after eight years in the Oval Office.
In his parting message to the American people, he said: "I think we're going to be okay."
Mr Obama is heading to the Californian desert resort of Palm Springs with former first lady Michelle Obama and with America assessing his legacy.
The future of the Obamacare healthcare reforms and executive actions on immigration appear to be at risk.
As the inauguration events continue, more than million anti-Trump protesters are expected to stage mass protests in Washington and around the country on Saturday.
The Women's March on Washington is aimed at highlighting Mr Trump's history of misogynistic comments and treatment of women.
The tycoon, who has never held public office before in his life, is now the most powerful leader on the planet.
Mr Trump begun his inauguration day with a visit to a church in Washington, and emerged from Blair House, a government guest house next to the White House, with Melania just after 1.30pm Irish time.
They then took a motorcade for the short journey to St John's Episcopal Church, where they were also joined by Mr Trump's adult children.
Before emerging for the church service, Mr Trump wrote on Twitter: "It all begins today! I will see you at 11:00 A.M. for the swearing-in. THE MOVEMENT CONTINUES - THE WORK BEGINS!"
It all begins today! I will see you at 11:00 A.M. for the swearing-in. THE MOVEMENT CONTINUES - THE WORK BEGINS!— Donald J. Trump (@realDonaldTrump) January 20, 2017
Mr Trump's defeated opponent Hillary Clinton, who polled nearly three million more votes but lost the Electoral College in November's election, is attending the ceremony along with her husband, former president Bill Clinton.
I'm here today to honor our democracy & its enduring values. I will never stop believing in our country & its future. #Inauguration— Hillary Clinton (@HillaryClinton) January 20, 2017
Security for the inauguration is likely to cost around US$100m (€93.8m), with 28,000 uniformed personnel working under the direction of the US Secret Service.
Up to a million spectators are thought to have watched the day's events, including a parade in which the new President traditionally walks at least part of the route back to the White House.
Supporters of Mr Trump started lining up at security checkpoints before dawn to take their places on the National Mall.