Hundreds of thousands of people poured into Washington to watch Donald Trump being sworn in as the 45th US president.
All eyes were on the US Capitol as Mr Trump formally replaced President Barack Obama in the White House.
Here is a guide to the historic transition.
What is a presidential inauguration?
The newly elected US president is sworn into office by the Chief Justice of the United States every four years by noon (17:00 GMT) on 20 January, as prescribed by the US Constitution.
Historically, the incoming president was inaugurated on 4 March, but the period of delay was shortened when the 20th Amendment was ratified in 1933.
The oath is part of a ceremony marking the peaceful transition of power on the steps in front of the US Capitol.
The ceremony is then followed by a parade down Pennsylvania Avenue and later celebrated through a series of inaugural balls.
What is happening today?
1. Trump leaves from Blair House on Friday morning 2. St John's Episcopal Church for morning service 3. White House coffee with Obama 4. US Capitol for Oath of office and address 5. National Mall, where spectators watch parade 6. Trump walks past his hotel as he leads the parade to his new home
FRIDAY, 20 JANUARY
- Mr Trump attended a morning service at St John's Episcopal Church near the White House
- Mr Trump and his wife, Melania, had coffee with President Obama and First Lady Michelle Obama.
- 10:30 (15:30 GMT) The couples took a motorcade to the Capitol. Inauguration ceremony began with musical performances
- 11:30 (16:30 GMT) Opening remarks followed by Supreme Court Justice swearing in Mr Pence
- 11:45-12:00 (16:45 - 17:00 GMT) President Obama's term of office ended at precisely noon. Mr Trump took the oath of office, administered by Chief Justice Roberts, at 12:00 (17:00 GMT). He then delivered his inaugural address
- 13:00-15:45 (18:00-20:45 GMT) President Trump signed a series of documents in a room at the US Capitol before dining with lawmakers at the inaugural luncheon
- 15:45-17:00 (20:45-22:00 GMT) Mr Trump and Mr Pence embarked on a 1.5 mile (2.4km) parade down Pennsylvania Avenue to the White House
- 19:00-23:00 (00:00-04:00 GMT) Mr Trump, Mr Pence and their wives will attend three official inaugural balls
SAT, 21 JANUARY
- 10:00 (15:00 GMT) Mr Trump and Mr Pence attend the interfaith National Prayer Service at the Washington National Cathedral
- 10:00 (15:00 GMT) The Women's March on Washington begins
Who is there?
President Obama and the first lady accompanied Mr Trump in a motorcade to the US Capitol for the official ceremony, where they were joined by members of Congress, politicians as well as supporters.
Former President Bill Clinton and Hillary Clinton, who lost to Mr Trump in November's election, are attending.
George W Bush and his wife, Laura, as well as Jimmy Carter, are also there.
George HW Bush, who was in hospital for respiratory problems, wrote a letter to Mr Trump wishing him well and apologising for missing the event due to health concerns.
An estimated 800,000 to 900,000 people were expected to flood the nation's capital on Friday for the inauguration, but it is unclear whether they will be there in celebration or protest, officials said.
President Barack Obama drew an estimated 1.8 million people to Washington when he took office eight years ago.
The "level of enthusiasm" and demand for hotel rooms has not reached that of previous inaugurations, according to Elliott Ferguson, president of Destination DC, the city's convention and tourism bureau.
In fact, some hotels have reduced the minimum-night stay from four nights to two.
Other hotels are only 50% full, but higher-end hotels appeared to have more bookings, he added.
Who is not going?
More than 50 House Democrats are publicly refusing to attend the ceremony amid a feud between the newly elected president and the civil rights activist and congressman, John Lewis.
Mr Lewis is among the congressmen who will not be in attendance. Some lawmakers have said they will instead attend the Women's March on Washington, a protest set to take place a day after the inauguration.
What are the protests about?
Several demonstrations both protesting and supporting Mr Trump will take place around the city over the weekend.
Most notably, the Women's March on Washington is estimated to draw crowds of 200,000 people on 21 January.
It sets out to demonstrate for racial and gender equality, affordable healthcare, abortion rights and voting rights - issues perceived to be under threat from a Trump presidency.
The motorcycle group Bikers for Trump will also host a rally for the incoming president after the ceremony and before the inauguration parade.
Other protests include:
- Anti-war and anti-nuclear weapons rally attended by former Green Party presidential candidate Jill Stein
- #DisruptJ20 Festival of Resistance, organised by the DC Counter-Inaugural Welcoming Committee
- #Trump420 march, hosted by marijuana advocates who plan to hand out 4,200 free joints (which is legal in Washington)
How is Mr Trump celebrating?
Mr Trump has enlisted the Mormon Tabernacle Choir, Radio City Rockettes, country stars Toby Keith and Lee Greenwood, 3 Doors Down as well as America's Got Talent contestant and singer Jackie Evancho to perform over the course of two days.
Academy Award-winning actor Jon Voight also spoke at the welcome concert, telling the crowd: "God answered all our prayers" by electing Mr Trump.
Evancho, 16, who took second place on the talent show, sang the national anthem.
The Trump transition team had reportedly struggled to secure high-profile entertainers.
A number of artists turned down the opportunity to play, including Elton John, Welsh singer Charlotte Church and American DJ Moby.
Dreamgirls star Jennifer Holliday announced she had dropped an inauguration performance to stand in solidarity with the LGBT community while a Bruce Springsteen tribute band also decided to skip the event out of respect for the Boss, who is vocally critical of Mr Trump.
The newly sworn-in president and vice-president will also stop by three official inaugural balls, which are typically attended by the incoming administration's supporters.
When does he start work?
The soon-to-be 45th president has vowed to roll back many of Mr Obama's policies on "day one" in office, but it is unclear whether that begins on Friday or Monday.
The new White House press secretary, Sean Spicer, tweeted that the president was signing three things in a ceremony at an ornate room within the US Capitol: a waiver bill enabling Gen James Mattis to serve as Defence Secretary, the formal nominations to go to the Senate, and a proclamation for a national "day of patriotism".
But Mr Trump had told the Times earlier this week that "day one" would not begin until Monday.
"I mean my day one is going to be Monday because I don't want to be signing and get it mixed up with lots of celebration," he told the newspaper.
How can I follow all this on the BBC?
To follow live coverage online, head to BBC News.
BBC World News will broadcast rolling television coverage from 14:00 to 23:00 GMT (09:00 until 18:00 EST) on Friday.
Our special broadcast presented by Katty Kay in Washington will carry the event live, and feature analysis and international reaction throughout the day.
Audiences in the UK can watch a News Channel special between 15:00 and 22:00 GMT.
BBC World Service radio will also be in Washington, broadcasting rolling coverage from 16:00 until 22:00 GMT (11:00 until 17:00 EST).