(AP) -- Hillary Clinton's campaign joined forces with former primary rival Bernie Sanders and some of his supporters Monday in a scramble to tamp down a fresh burst of party disunity and give an upbeat tone to the opening night of the Democratic convention.

Sanders, one of the night's featured speakers, sent urgent messages to his backers urging them to avoid protests on the convention floor. The Clinton campaign opened up speaking spots for his supporters who touted his accomplishments, and warned that not backing Clinton would only help Republican Donald Trump.

An array of office holders and celebrities hammered home the call for unity, with singer Paul Simon singing his "Bridge Over Troubled Water" as delegates linked arms and swayed to the music.

Former President Bill "Clinton smiled and clapped from the audience.

First lady Michelle Obama was making her first appearance of the presidential campaign. Massachusetts Sen. Elizabeth Warren, a favorite of liberals and one of the party's toughest critics of Trump, was also taking the stage.

"Trump thinks he can win votes by fanning the flames of fear and hatred," Warren said in excerpts released ahead of her speech. "By turning neighbor against neighbor. By persuading you that the real problem in America is your fellow Americans -- people who don't look like you, or don't talk like you, or don't worship like you."

Clinton's campaign hoped the nighttime line-up would overshadow a tumultuous start to the four-day convention. The release of hacked party emails revealed the Democratic National Committee had favored Clinton over Sanders in the primary, despite vows of neutrality. The uproar led to the forced resignation of party chair Debbie Wasserman Schultz.

While her ouster was a major victory for Sanders, it wasn't enough to ease the frustration of his supporters. Inside the arena, chants of "Bernie" echoed through the arena as the convention opened, and boos could be heard at times when Clinton's name was raised. Outside the convention hall, several hundred Sanders backers marched down Philadelphia's sweltering streets with signs carrying messages such as "Never Hillary."

Campaigning in North Carolina, Trump seemed to revel in the Democrats' commotion, telling supporters that Clinton made a mistake by not choosing a more liberal running mate to appease Sanders' base. "Crazy Bernie's going crazy right now," he said.

But in Philadelphia, Delegates waved "Love Trumps Hate" signs and leapt to their feet as immigration supporters, gay rights advocates, and labor leaders took the stage.

Comedian-turned-Sen. Al Franken, a Clinton supporter, and actress Sarah Silverman, a Sanders supporter, 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."

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 Clinton was bitingly personal, including chants of "Lock her up."

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 from Sanders' backers.

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

Clinton's team hoped Wasserman Schultz's resignation -- along with an apology from the DNC to Sanders and his supporters -- would keep the convention floor calm.

Discussions between the two camps prompted Sanders to send emails and text messages to supporters asking them not to protest.

"Our credibility as a movement will be damaged by booing, turning of backs, walking out or other similar displays," Sanders wrote.

The party infighting had echoes of last week's Republican convention, where some major GOP leaders voiced their displeasure with Trump and others didn't even show up. Clinton promised a stark contrast to the GOP gathering, saying she planned to highlight "success stories" and flesh out details of her proposed policies.

Sanders was a relatively unknown Vermont senator when he decided to challenge for the Democratic nomination. He stunned the Clinton campaign with his broad support among young people and liberals, as well as his online fundraising prowess. But he struggled to appeal to black voters and couldn't match the former secretary of state's ties to the Democratic establishment.

Sanders previewed his remarks during an appearance earlier Monday before supportive delegates. He implored them to vote for Clinton, generating a chorus of boos.

"Brothers and sisters, this is the real world that we live in," Sanders said as he tried to quiet the crowd. "Trump is a bully and a demagogue."

The controversy over some 19,000 leaked DNC emails, however, threatened to complicate those plans. The correspondence, posted by WikiLeaks over the weekend, showed top officials at the supposedly neutral DNC favoring Clinton over Sanders in the presidential primaries.

Clinton campaign officials blamed the hack, which is now being investigated by the FBI, on Russian military intelligence agencies. The campaign also accused Moscow of trying to meddle in the U.S. election and help Trump, who has said he might not necessarily defend NATO allies if they are attacked by Russia.

A cybersecurity firm the Democrats employed found traces of at least two sophisticated hacking groups on their network -- both of which have ties to the Russian government. Those hackers took at least a year's worth of detailed chats, emails and research on Trump, according to a person knowledgeable of the breach who wasn't authorized to speak publicly about the matter.

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Associated Press writers Kathleen Hennessey, Catherine Lucey, Kathleen Ronayne and Julie Bykowicz in Philadelphia, Lisa Lerer in Charlotte, North Carolina, contributed to this report.

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