Michelle Obama has endorsed Democratic presidential hopeful Hillary Clinton to be the next US president in a rousing speech at the Democratic National Convention in Philadelphia.

The First Lady enthusiastically gave her backing to Mrs Clinton, saying to rapturous applause that the former Secretary of State is the only person "truly qualified to be president of the United States".

"When crisis hits, we don't turn against each other, we listen to each other, we lean on each other because we are always stronger together," Mrs Obama said.

"And I am here tonight because I know that that is the kind of president that Hillary Clinton will be and that's why in this election, I'm with her."

Crowds cheered and echoed her words with chants of "I'm with her".

The image of children was the predominant motif in the First Lady's speech, and she frequently referenced Mrs Clinton's record with promoting children's rights - as a young lawyer advocating for children with disabilities, fighting for child healthcare as First Lady, and "travelling the globe to keep our kids safe" as Secretary of State.

Mrs Obama said she admires the Democratic presidential frontrunner most because she never buckles under pressure, never takes the easy way out, and because she has "never quit on anything in her life".

"When I think about the kind of president I want for my girls and all our children, that's what I want," she said. "I want someone with the proven strength to persevere. Someone who knows this job and takes it seriously."

In a reference to Republican president hopeful Donald Trump's campaign slogan, Mrs Obama added: "Don't let anyone ever tell you that this country isn't great. That somehow, we need to make it great again. Because this right now, is the greatest country on earth."

"We need to pour every last ounce of our passion and our strength and our love for this country into electing Hillary Clinton as president of the United States of America."