US President Donald Trump left a letter to his successor Joe Biden before leaving the White House, spokesman Judd Deere told AFP.

Trump, who refused to accept his defeat for more than two months, never congratulated Joe Biden on his victory.

“We are just temporary occupants of this office,” Barack Obama wrote in his own note to Trump four years ago, left in the Oval Office in line with presidential tradition.

“That makes us guardians of those democratic institutions and traditions… that our forebears fought and bled for. Regardless of the push and pull of daily politics, it’s up to us to leave those instruments of our democracy at least as strong as we found them,” he added.

Meanwhile, Biden attended a mass at St Matthew’s Cathedral in Washington, DC on Wednesday morning, accompanied by Democratic and Republican leaders of Congress in a symbol of unity ahead of his inauguration.

The future 46th President of the United States was accompanied by his wife Jill Biden, Democratic House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer, as well as Republican Senate leader Mitch McConnell and House leader Kevin McCarthy.

Biden will be sworn in shortly before noon (1700 GMT) outside the Capitol, which was targeted in a deadly assault by pro-Trump protesters exactly two weeks ago.