President Donald Trump has painted a bleak picture of a broken country after being sworn in as US president.
He spoke of abandoned factories, rampant crime and a failed education system, pledging that his presidency would bring about change.
“This American carnage stops right here and stops right now,” President Trump said on the steps of the Capitol.
Thousands of Trump supporters traveled across the country to witness the occasion from the National Mall.
The moment marks the end of an improbable journey for the property tycoon after a campaign marked by controversy.
Shortly after the ceremony Mr Trump was seen signing his first official actions as the 45th president.
He sent his Cabinet nominations to the Senate as well as a signed a proclamation for a national day of patriotism, according to Press Secretary Sean Spicer.
He also signed into law a waiver allowing retired Marine General James Mattis, his pick for defence secretary, to serve in the post.
Mr Trump briefly spoke at a Capitol luncheon to thank his Democratic opponent Hillary Clinton and her husband, former President Bill Clinton.
He asked the former president and Mrs Clinton, who lost to Mr Trump in a dramatic upset in November’s election, to stand to be recognised. The pair rose to applause.
In his inaugural address, he promised to be the voice of the “forgotten people”, ignored by Washington politicians.
Today, he said, was “the day the people became rulers of this nation again”.
“I will fight for you with every breath left in my body and I will never ever let you down,” said President Trump after Chief Justice John Roberts administered the oath.
“America will start winning again, winning like never before.
“We will bring back our jobs, bring back our borders, bring back our wealth and we will bring back our dreams.”
President Trump and Vice-President Mike Pence waved goodbye as the Bidens and Obamas left the Capitol.
The Obamas held hands as they boarded a military helicopter that took them to Joint Base Andrews in Maryland.
Mr Obama delivered remarks to staff and supporters before he and his wife flew to Palm Springs, California, for a holiday.
He told a crowd they “proved the power of hope” and that “this isn’t a period, it’s a comma in the continuing story of building America”.
The historic moment drew congratulation messages from dignitaries around the world including Pope Francis, who said he was praying Mr Trump’s decisions would be guided by the “rich spiritual and ethical values” that have shaped America’s history.
British Foreign Secretary Boris Johnson also offered well wishes, and Canadian Prime Minister said he looked forward to restoring “prosperity to the middle class on both sides of the border”.
Former presidents and first ladies, including George W Bush and his wife Laura as well as Jimmy Carter, were in attendance.
The only absences were 92-year-old George Bush Senior, who is in hospital being treated for respiratory problems, and his wife Barbara.
Members of Congress were also in attendance, although more than 50 House Democrats had refused to attend the ceremony in protest.
Mr Trump takes power at a time when the country appears to be deeply divided. He enters the presidency with historically low approval ratings.
He has vowed to roll back many of his predecessor’s policies, including repealing Mr Obama’s signature health care law and building a wall along the US-Mexico border
Authorities arrested nearly 100 people protesting against the inauguration, according to the Metropolitan Police Department.
Many were apprehended for “vandalism and destruction of property”, said spokesman Lieutenant Sean Conboy.
Mr Conboy also said two police officers were hurt during clashes.
Earlier, about 150 protesters dressed in black marched through Washington, smashing windows and rolling rubbish bins into the street to form blockades.
The Women’s March on Washington on Saturday – for racial and gender equality, and other issues perceived to be under threat from Mr Trump’s administration – is expected to draw about 200,000 people.