Donald Trump is set to be sworn in as the 45th president of the United States.
The property tycoon won an unexpected victory in November's election after a controversial campaign.
Hundreds of thousands of supporters and protesters are travelling to Washington DC for the event.
Thousands of police have been deployed and many kilometres of crowd barriers set up as part of a vast security operation.
Addressing supporters on Thursday night at a pre-inaugural concert, Mr Trump pledged to unified America, bring change and make the country great "for all of our people".
Meanwhile in New York, thousands of people attended a rally where dozens of celebrities and politicians voiced their concerns about the president-elect.
Mr Trump will be sworn in before midday local time (17:00 GMT) by Chief Justice John Roberts, and make his inaugural speech.
The ceremony will be attended by former presidents and first ladies, including Bill Clinton and his wife Hillary, Mr Trump's opponent in the recent election.
The only absences will be 92-year-old George Bush Senior, who is in hospital being treated for respiratory problems, and his wife Barbara.
Members of Congress will also be there, although more than 50 House Democrats have said they will boycott the event.
An estimated 800,000 to 900,000 people are travelling to Washington, although it is not clear how many are coming to celebrate the inauguration and how many to protest against it.
Demonstrations for and against Mr Trump include a rally for the new president by Bikers for Trump after the ceremony.
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.