US First Lady Michelle Obama on Monday told the Democratic convention in Philadelphia that Hillary Clinton was the one candidate ready to be president, as she poured scorn on Donald Trump's behavior and temperament.

"In this election, there is only one person who I trust with that responsibility, only one person who I believe is truly qualified to be president of the United States, and that is our friend Hillary Clinton," she said to standing ovations.

"In this election, I'm with her."

Obama painted a picture of her husband's first secretary of state as a competent, steely politician who does not make "snap decisions" or judgments.

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

And she noted how Clinton, when she lost her Democratic nomination race to Barack Obama in 2008, "didn't get angry or disillusioned."

"As a true public servant, Hillary knows that this is so much bigger than her own desires and disappointments."


The first lady also turned personal, her voice cracking at times, when she discussed the future at stake for her two daughters.

"Because of Hillary Clinton, my daughters and all of our sons and daughters now take for granted that a woman can be president of the United States."

Obama's warm, eloquent remarks brought the house down, appearing to generate the unity that had eluded much of the first day's proceedings, when die-hard Bernie Sanders supporters booed and jeered speakers when they offered their support for Clinton.

Obama, whose president husband addresses the convention on Wednesday, took a veiled dig at Republican nominee Trump, whose campaign slogan is "Make America great again."

"Don't let anyone ever tell you that this country is not great," she said. "Because this right now is the greatest country on Earth."

The nation, she said, needs "a leader who will be guided every day by the love and hope and impossibly big dreams that we all have for our children."

Americans must not turn cynical by the rhetoric of this year's presidential election, she said.

"No, hear me," Obama added. "Between now and November... we need to pour every last ounce of our passion and our strength and our love for this country into electing Hillary Clinton."