President-elect Joe Biden declared it was "time to heal" America and a "nation has united, strengthened and healed" in his first speech.

"Biden won on Saturday in a bitter election, even as President Donald Trump refused to concede Biden's victory in the battleground state of Pennsylvania put him over the threshold of 270 Electoral College votes he needed to clinch the presidency, ending four days of nail-biting suspense and sending his supporters into the streets of major cities in celebration", news agency Reuters said.

"The people of this nation have spoken. They have delivered us a clear victory, a convincing victory," Biden said in his victory speech in his home town of Wilmington, Delaware.

"I pledge to be a president who seeks not to divide, but to unify," he said, then addressed Trump's supporters directly.

"Now, let's give each other a chance. It's time to put away the harsh rhetoric, lower the temperature, see each other again, listen to each other again," he said.

Biden also tweeted:

"This is the time to heal in America." He was introduced by his running mate, US Senator Kamala Harris, who will be the first woman, the first Black American and the first American of Asian descent to serve as vice president, the country's No 2 office.

"What a testament it is to Joe's character that he had the audacity to break one of the most substantial barriers that exists in our country, and select a woman as his vice president," Harris was quoted stating in the Reuters report.