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US first lady Michelle Obama has denounced Republican Donald Trump and passionately backed Hillary Clinton at the Democratic National Convention in Philadelphia.

"The hateful language... from public figures on TV does not represent the true spirit of this country," she said, to loud cheers and applause.

"Don't stoop to their level. Our motto is, when they go low, we go high."

Earlier, Vermont Senator Bernie Sanders urged Democrats to back Clinton.

She will accept the party's presidential nomination on Thursday.

"While Donald Trump is busy insulting one group after another, Hillary Clinton understands that our diversity is one of our greatest strengths," Sanders said.

Some supporters of the senator, who had been Clinton’s rival in the Democratic primary, booed mention of Clinton’s name; others wore duct tape with the word "silenced" over their mouths.

 Obama's speech was widely seen as an attack on Trump, although she did not mention the Republican nominee by name.

She said she wanted her husband to be succeeded by "somebody who knows this job and takes it seriously".

"Somebody who understands that the issues of our nation are not black or white. It cannot be boiled down to 140 characters," she said, in a reference to Trump’s enthusiasm for using Twitter.

"You can't have thin skin or a tendency to lash out. You need to be steady and measured and well-informed."

She said there was "only one person who I trust with that responsibility, only one person who I believe is truly qualified to be president... and that is our friend Hillary Clinton".

Because of the Democrats' candidate, she said, "my daughters and all our sons and daughters now take for granted that a woman can be president of the United States."

In other remarks, she praised Clinton as a "true public servant" who had shown "devotion to our nation's children" and "never buckles under pressure".

After her speech, President Obama wrote on Twitter: "Incredible speech by an incredible woman. Couldn't be more proud & our country has been blessed to have her as FLOTUS [first lady]. I love you, Michelle."

Garry Mauro, head of the Clinton delegation for Texas, said Obama’s speech was "stunningly good."

Trump, who has criticised his rivals, both Republican and Democrat, in sometimes colourful language on Twitter, said Sanders had "sold out" to Clinton.

He also condemned other speakers at the Democratic convention including senators Cory Booker and Elizabeth Warren, but made no mention of Obama.

Five times Michelle Obama referred to Donald Trump

-"When someone is cruel or acts like a bully, you don't stoop to their level''

-"We urge them [our daughters] to ignore those who question their father's citizenship or faith"

-"Don't let anyone ever tell you that this country isn't great"

-Issues facing a president "cannot be boiled down to 140 characters"

-Ideal president "can't have a thin skin or a tendency to lash out"

In other key moments from the convention on Monday

Senator Warren attacked the business record of Trump

Comedian Sarah Silverman, who supported Sanders, told his supporters to get behind s Clinton

One of the loudest cheers went to Senator Cory Booker when he told the crowd: "In America, love always trumps hate"