Joe Biden said Monday it is "time to turn the page" after his presidential election victory was confirmed by the US Electoral College.
In a speech after the announcement, he said US democracy had been "pushed, tested and threatened" and "proved to be resilient, true and strong", BBC reported.
He condemned President Donald Trump's attempts to overturn the result.
Later Russian President Vladimir Putin became one of the last world leaders to congratulate Biden on his victory.
Moscow had said it would wait for the official results before doing so. Most other national leaders contacted Biden days after the vote on November 3.
Confirmation by the Electoral College was one of the steps required for Biden to take office.
Under the US system, voters actually cast their ballots for "electors", who in turn formally vote for candidates after the election.
Democrat Biden won November's contest with 306 Electoral College votes to Republican Trump's 232.
Speaking in Delaware, Biden praised "ordinary men and women" who had refused to be bullied, referring to the president's efforts to question and overturn the results, involving legal challenges which have been rejected by courts across the country.
He described the efforts as "a position so extreme we've never seen it before".
"Respecting the will of the people is at the heart of our democracy, even when we find those results hard to accept," he said.
"The flame of democracy was lit in this nation a long time ago," he added. "And we know that nothing not even a pandemic or an abuse of power can extinguish that flame."
Biden said it was time to "turn the page, as we've done throughout our history, to unite, to heal".
He described the harassment of officials following the election as "unconscionable" and said: "It's my sincere hope we never again see anyone subjected to the kind of threats and abuse we saw in this election."
He also noted that he had the same number of Electoral College votes that Trump said was a "landslide" when he won in 2016. Biden emphasized that he had also won the popular vote, something Trump failed to clinch four years ago.
Trump announced that Attorney General William Barr, one of his staunchest allies, will step down before Christmas.
Barr's term was due to end on January 20, when Trump leaves office.
Tensions between the two flared after Barr said there was no evidence of widespread fraud in November's vote.
He was criticized by Trump for not publicly disclosing during the election campaign that the Justice Department was investigating Biden's son.
Trump tweeted Barr's resignation letter saying: "Our relationship has been a very good one, he has done an outstanding job!"
Deputy Attorney General Jeff Rosen will serve as acting attorney general, Trump said.