Bernie Sanders and Michelle Obama try to heal wounds in Democratic Party

"Any objective observer will conclude that, based on her ideas and her leadership, Hillary Clinton must become the next president of the United States," Mr Sanders declared in a headlining address on the opening night of the Democratic convention in Philadelphia.

Mr Sanders joined a high-wattage line-up of speakers, including Michelle Obama, whose address all but wiped away earlier tumult in the convention hall that had exposed lingering tensions between Clinton and Sanders supporters.

Mrs Obama, who has spent nearly eight years in the White House avoiding political fights, took numerous swipes at Mr Trump, while avoiding mentioning him by name.

"This election and every election is about who will have the power to shape our children for the next four or eight years of their lives," she said.

"There is only one person I trust with that responsibility, only one person I believe is truly qualified to be president of the United States, and that is Hillary Clinton."

While Mr Sanders had endorsed Mrs Clinton previously, his remarks early on Tuesday marked his most vigorous and detailed praise of her qualifications for the presidency.

It came at a crucial moment for Mrs Clinton's campaign, on the heels of leaked emails suggesting the party had favoured the former secretary of state through the primaries despite a vow of neutrality.

Mr Sanders scored the resignation of party chairwoman Debbie Wasserman Schultz, a nemesis in the primaries - but that was not enough to quell the anger of supporters.

As the convention opened, they still erupted in chants of "Bernie" and booed Mrs Clinton the first several times her name was mentioned.

Outside the convention hall, several hundred marched down Philadelphia's sweltering streets with signs carrying messages such as "Never Hillary".

Behind the scenes, Sanders and Clinton aides joined forces to try to ease tensions.

Mrs Clinton's campaign quickly added more Sanders supporters to the speakers line-up and Mr Sanders sent urgent messages asking them not to protest.

By the time Mr Sanders took the stage for the night's closing address, much of the anger had been overshadowed by speeches promoting party unity.

He did his part, imploring his supporters to consider a country under Mr Trump's leadership.

"If you don't believe this election is important, if you think you can sit it out, take a moment to think about the Supreme Court justices that Donald Trump would nominate and what that would mean to civil liberties, equal rights and the future of our country," he said.

Mrs Clinton's husband, former US president Bill Clinton, watching from the audience, leapt to his feet and applauded, as did most of the delegates filling the convention arena.

Mr Sanders spoke just after Massachusetts senator Elizabeth Warren, a favourite of liberals who has emerged as one of the Democrats' toughest critics of Mr Trump.

"Donald Trump has no real plans for jobs, for college kids, for seniors," she said in the keynote address. "No plans to make anything great for anyone except rich guys like Donald Trump."

Mrs Obama was one of the night's stand-outs. While she has often avoided overt politics in the White House, her frustration with Mr Trump's rise was evident.

She warned that the White House could not be in the hands of someone with "a thin skin or a tendency to lash out" or someone who tells voters the country can be great again.

"This right now, is the greatest country on earth," she said.

Mrs Clinton's campaign hoped the night-time line-up would overshadow a tumultuous start to the four-day convention.

The hacked Democratic National Committee emails fed the suspicion of Mr Sanders' supporters and sapped Mrs Clinton's campaign of some of its energy following a well-received roll-out on Saturday of her running mate, Virginia senator Tim Kaine.

Campaigning in North Carolina, Mr Trump seemed to revel in the Democrats' commotion, telling supporters that Mrs Clinton made a mistake by not choosing a more liberal running mate to appease Mr Sanders' base.

"Crazy Bernie's going crazy right now," he said.

But in Philadelphia, delegates waved "Love Trumps Hate" signs and cheered as immigration supporters, gay rights advocates, and union leaders took the stage.

Comedian-turned-senator Al Franken, a Clinton supporter, and actress Sarah Silverman, a Sanders backer, made a joint appearance to promote party unity.

"I am proud to be part of Bernie's movement," Silverman said as the crowd roared. "And a vital part of that movement is making absolutely sure Hillary Clinton is our next president of the United States."

Mr Trump was a frequent target throughout the night, though the jabs were often more mocking than mean. The tone was a sharp contrast to the Republican convention, where the attacks against Mrs Clinton was bitingly personal, including chants of "Lock her up".

Ms Wasserman Schultz had planned to be among those taking the stage, despite the email hacking controversy. But she stepped aside, bowing to pressure from Democrats who feared the mere sight of her on stage would prompt strong opposition.

The outgoing chairwoman watched the gathering from a private suite at the arena.