Supporters of Bernie Sanders have disrupted the first day of the Democratic convention, repeatedly chanting and booing mentions of Hillary Clinton's name as the party's hopes for a show of unity dissolved into frequent chaos.
Speakers in the convention's first hour struggled to carry out business as angry Sanders supporters roared their disapproval, drawing a deafening response from Clinton delegates.
"We're all Democrats and we need to act like it," US Representative Marcia Fudge of Ohio, the convention's chairwoman, shouted over the uproar.
Earlier in the day, Sanders drew jeers from his supporters when he urged his delegates to back the White House bid of his formal rival, Clinton, and focus on defeating Republican Donald Trump in the November 8 presidential election.
Sanders' followers shouted: "We want Bernie" in a show of anger at both Clinton's victory in the race for the Democratic presidential nomination and emails leaked on Friday suggesting the party leadership had tried to sabotage Sanders' insurgent campaign.
For months, Sanders, 74, a US senator from Vermont, mounted an unexpectedly tough challenge to Clinton, 68, a former secretary of state, who this week will become the first woman nominated for president by a major US political party.
The scenes of booing in Philadelphia were a setback to Democratic officials' attempts to present the gathering as a smoothly run show of party unity in contrast to the volatile campaign of Republican nominee Trump.
Sanders tried to head off the disruptions, sending an email to delegates as the convention opened, urging them to refrain from interrupting the proceedings.
"Our credibility as a movement will be damaged by booing, turning of backs, walking out or other similar displays. That's what the corporate media wants. That's what Donald Trump wants," Sanders said in the email.
Trump gloated at the Democrats' opening day disorder.
"Wow, the Republican Convention went so smoothly compared to the Dems total mess," he wrote on Twitter.
The Democratic National Committee formally offered "a deep and sincere apology to Senator Sanders, his supporters, and the entire Democratic Party for the inexcusable remarks made over email" and said it would take action to ensure it never happens again.
Democratic National Committee chairwoman Debbie Wasserman Schultz resigned effective at the end of the convention over the email flap.
At a morning gathering of Florida delegates, Sanders supporters jeered Wasserman Schultz, who they accuse of trying to sabotage the campaign of the democratic socialist.
Sanders, speaking later to his delegates in Philadelphia, was booed when he urged supporters to help defeat Trump by backing Clinton and her vice presidential running mate, US Senator Tim Kaine of Virginia.
"Brothers and sisters, this is the real world that we live in," he said, adding: "Trump is a bully and a demagogue."
Members of the crowd screamed back: "So is Hillary."
"She stole the election!" someone else shouted.
While Sanders has endorsed Clinton, the former first lady faces a difficult task attracting his backers as she battles Trump.
The New York businessman pulled ahead in at least one opinion poll on Monday, after lagging Clinton in most national surveys for months.
A CNN/ORC opinion poll gave Trump a 48 per cent to 45 per cent lead over Clinton in a two-way presidential contest.
Trump was formally nominated for president at a chaotic Republican convention in Cleveland last week.
Sanders was among those due to speak on the first evening of the Democratic convention. Other speakers included President Barack Obama's wife, Michelle Obama.
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