Washington DC - President Joe Biden told Americans Wednesday that he had dropped out of the 2024 election to unite his party and his country, saying in a historic Oval Office speech that it was time to pass the torch to "younger voices."

In his first televised address since his stunning decision, the 81-year-old hailed his "tough" and "capable" Vice President Kamala Harris (59), who is set to be the new Democratic presidential nominee.

"The defense of democracy, which is at stake, is more important than any title," Biden said.

"I have decided the best way forward is to pass the torch to a new generation. That is the best way to unite our nation."

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Biden bowed out of the race later than any other president in US history on Sunday, giving in to weeks of pressure from Democrats after a disastrous debate performance against Donald Trump raised concerns about his age and mental acuity.

Using the powerfully symbolic setting of the Resolute Desk, Biden's speech called for an end to the divisions in US politics and said the country was more powerful than "any dictator or tyrant."

The veteran Democrat said there was a "time and a place for new voices, fresh voices. Yes, younger voices. And that time and place is now."

Biden also rejected accusations that he would be a lame duck for the remaining six months of his one-term presidency.

"Over the next six months I will be focused on doing my job as president," he said, adding that he would keep trying to lower costs for families and defend freedoms including the right to abortion.

Biden's withdrawal has upended Trump's campaign, which had previously focused on the president's age and mental acuity. Now it is Trump (78), who is the oldest candidate in US history.