Joe Biden has been sworn in as the 46th US president, ending one of the most dramatic political transitions in American history.
"Democracy has prevailed," he said after taking the oath of office from Chief Justice John Roberts.
Donald Trump - who has not formally conceded the presidency to Mr Biden - snubbed the inauguration ceremony, in a departure from longstanding precedent.
The new president has announced a raft of measures reversing Trump policies.
Kamala Harris was sworn in as vice-president ahead of Mr Biden. She is the first woman - and the first black and Asian-American person - to serve in a role, a heartbeat from the presidency.
The inauguration took place at the US Capitol. There is extra-tight security after the building was stormed by violent pro-Trump protesters in a deadly riot on 6 January.
Some 25,000 National Guards protected the ceremony, which missed the traditional hundreds of thousands of spectators because of the coronavirus pandemic.
Photo BBC Caption: 46th US President Joe Biden