Democratic party heavyweights Michelle Obama and Bernie Sanders have offered contrasting heartfelt and hard-headed endorsements of Hillary Clinton, imploring a riven and feisty party convention to unite to stop Donald Trump.

Key points: Mr Sanders said his former rival "must become the next president"

Mr Sanders said his former rival "must become the next president" Sanders supporters jeered throughout day one of the event

Sanders supporters jeered throughout day one of the event Michelle Obama's speech earned the most positive result of the night

As polls showed Mr Trump ahead of Mrs Clinton in the race to the White House, the First Lady wowed the Philadelphia crowd as she impeached Mr Trump's behaviour and hailed the inspirational power of possibly having a first female US president.

From Mr Sanders, Mrs Clinton's vanquished primary rival, there was a much more pragmatic embrace.

"Based on her ideas and her leadership", Mrs Clinton was a better choice than Mr Trump and "must become the next president of the United States", Mr Sanders said.

The opening day of the four-day convention in Philadelphia was dominated by boos and jeers from disgruntled supporters of Mr Sanders almost every time Mrs Clinton's name was mentioned.

Mr Sanders had called on his supporters to get behind the Democratic nominee twice on Monday before his primetime endorsement speech.

That included a text message sent to supporters asking them not to protest on the floor of the convention as a "personal courtesy" to him.

But his self-styled "political revolution" appeared to have transformed into a revolt.

Mr Sanders himself was booed by some sections of the audience when he told the crowd: "Hillary Clinton will make an outstanding president and I am proud to stand with her tonight."

As the boos and chants of his name continued inside the convention hall throughout the evening, Mr Sanders protesters outside tried to breach security barriers, leading to 54 people being briefly detained and fined $50 each.

"Clinton can't beat Trump. Period," said Michigan delegate Melissa Arab, a supporter of Mr Sanders.

"A ham sandwich could beat Trump and she's not going to beat him. If she's nominated, people are going to end up with somebody bad for president."

Hoping to poach some of Mr Sanders' supporters, Mr Trump tweeted: "Bernie Sanders totally sold out to Crooked Hillary Clinton. All of that work, energy and money, and nothing to show for it. Waste of time."

Supporters of Senator Bernie Sanders react as they listen to him speak during the Democratic National Convention. ( Reuters: Rick Wilking )

'This right now is the greatest country on Earth'

Mrs Obama's message was at once conciliatory, raw and personal — and earned by far the most positive response of the night.

Sorry, this video has expired Michelle Obama says the US needs a president that's measured and well-informed

"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," said the wife of America's first black president, her voice cracking with emotion.

Mrs Obama described America as "the greatest country on Earth", clashing with Republican Donald Trump's "Make America Great Again" slogan and bringing an arena crowd riven by a fierce fight for the nomination to its feet.

"I wake up every morning in a house that was built by slaves, and I watch my daughters — two beautiful, intelligent, black young women — playing with their dogs on the White House lawn," Michelle Obama said.

"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," she said.

The outgoing First Lady reminisced about her two "bubbly little girls" Sasha and Malia as they entered the White House, and how they are leaving it as "poised young women".

But — in a thinly veiled jab at Mr Trump — she also painted a picture of a family that had to struggle with the shrill tone of today's zero-sum politics.

"We urge them to ignore those who question their father's citizenship or faith," she said, a clear reference to Mr Trump's early demands to see President Barack Obama's birth certificate, Mrs Obama said.

"We insist that the hateful language they hear from public figures on TV does not represent the true spirit of this country."

"Our motto is, 'when they go low, we go high,'" the First Lady said in a message that Democrats will hope resonates with fathers and mothers voting in November.

Mrs Obama did not always have easy relations with Team Clinton during the 2008 primary race, but she lauded the former first lady for not getting angry when she lost to Mr Obama that year.

"Hillary did not pack up and go home. Because as a true public servant, Hillary knows that this is so much bigger than her own desires and disappointments," she said.

AFP