US Vice-President Joe Biden has announced he will not run for the Democratic nomination in the 2016 White House race. Photo: www.crowdpac.com

Mr Biden said his family was ready after the death of his son earlier this year, but he had now run out of time.

He said it would be a mistake for Democrats to turn their backs on President Barack Obama's record.

Democrats seeking an alternative to frontrunner Hillary Clinton had been urging the 72-year-old to run.

Though he will not be a candidate, Mr Biden said he "will not be silent".

"I intend to speak out clearly and forcefully on where we stand as a party and where we need to go as a nation."

He said any candidate would be making a "tragic mistake" to reject the Obama legacy, and urged an end to political bickering.

"I believe we have to end the divisive, partisan politics ripping apart this country," he said as he stood in the Rose Garden of the White House, flanked by his wife Jill and Mr Obama.

Repeating a dig at Mrs Clinton that he has made several times this week, he said it was wrong to see Republicans as enemies.

When asked at last week's Democratic debate, the former first lady said she was proud of making an enemy out of Republicans.

Analysis - Anthony Zurcher, BBC North America reporter

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In the end, cold political reality won out. As Mr Biden acknowledged during his speech, there just wasn't enough time to mount a successful bid for the presidency.

Could there have been a moment of opportunity if he had announced back in August, when speculation of a Biden candidacy first surfaced? We will never know. But no matter when he made his decision, it was always going to be an uphill climb.

Hillary Clinton, despite her stumbles over the last few months, is a formidable candidate. She has raised tens of millions of dollars for her campaign, she has institutional support across the Democratic Party and she's been positioning herself for this race since 2008, when was edged out by Barack Obama.

During his Rose Garden speech, Mr Biden defended Mr Obama's presidential legacy and took another veiled shot at Mrs Clinton's remark last week that she considered Republicans to be her enemies.

He said he will continue to speak out over the course of the campaign, but today - given the rapt attention he commanded as a potential candidate - was his valedictory address.

His son Beau died from brain cancer in May, another family tragedy for the former Delaware senator after the deaths of his baby daughter and first wife in 1972.

He then rose through the Senate ranks and ran for president in 1988 and 2008.

After hearing the news, presidential candidates for 2016 tweeted their well wishes.

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Mr Sanders also tweeted that he supported Mr Biden's plans to make college free, fight economic inequality and close tax loopholes.

Democratic candidate Martin O' Malley tweeted that he respected Mr Biden's decision and that he is "one of the most decent, compassionate public servants our nation has produced".