Democrat Joe Biden on Saturday pledged to be a "president for all Americans," after US networks projected he has defeated Republican incumbent Donald Trump in their bitterly contested election.

"The work ahead of us will be hard, but I promise you this: I will be a President for all Americans -- whether you voted for me or not," the 77-year-old former vice president said in a tweet.

"With the campaign over, it's time to put the anger and the harsh rhetoric behind us and come together as a nation," he said in a separate statement. "It's time for America to unite. And to heal."

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