Bernie Sanders has urged Democrats to put Hillary Clinton in the White House, in a speech to the party convention.
The Vermont senator received a three-minute standing ovation when he took the stage in Philadelphia.
"Hillary Clinton must become the next president of the United States. The choice is not even close," he said.
Earlier in the evening, his fans had booed any mention of Hillary Clinton, who will accept the party's presidential nomination on Thursday.
And as he implored them to back his Democratic rival in the final major speech of the night, they held aloft their blue "Bernie" signs and chanted his name.
In other highlights on Monday:
Loudest cheer of the night went to New Jersey Senator Cory Booker when he told the crowd: "In America, love always trumps hate"
First Lady Michelle Obama was given a rapturous reception after a speech with several references to Republican Donald Trump
She said: "When someone is cruel or acts like a bully, we don't stoop to their level.''
Senator Elizabeth Warren attacked the business record of Republican Donald Trump
Comedian Sarah Silverman, who supported Mr Sanders, told his supporters to get behind Mrs Clinton
Members of the audience disrupted the event's opening prayer, chanting "Bernie!" while also jeering as DNC Chair Marcia Fudge delivered opening remarks.
Mr Sanders sent an email saying the credibility of the progressive movement would be damaged by "booing, turning of backs, walking out or other similar displays" of protests.
"That's what Donald Trump wants," he wrote. "We have made great progress in the last year. Let's continue going forward."
Revelations from an email leak which showed DNC officials allegedly plotted against Mr Sanders' primary campaign appeared to overshadow any sentiment of party unity.
Wikileaks released emails that revealed the DNC was biased against Mr Sanders when he ran against Mrs Clinton in the hard-fought primary contest.
The FBI has confirmed that it is investigating the leak.
Chairwoman Debbie Wasserman Schultz resigned on Sunday as pressure built on the party to address the scandal. The outgoing chair said on Monday she would no longer be speaking at the opening of the convention.
Then Democratic party bosses issued an apology to Mr Sanders for "inexcusable" emails which tried to undermine his White House campaign.
A few hours earlier, Mr Sanders had implored his supporters to defeat Donald Trump by getting behind Mrs Clinton and her running mate Tim Kaine.
His endorsement was met by loyal supporters in the crowd jeering loudly and shouting: "We want Bernie!"
He said his unexpectedly successful candidacy had proved that Americans "want a bold, progressive agenda that takes on the billionaire class".
Democrats were hoping that a cohesive convention would draw a sharp contrast with the sometimes chaotic Republican gathering last week. Bernie Sanders, who recently endorsed Hillary Clinton, will reportedly do his part, calling for party unity in his convention address.
For the moment, however, many of the senator's delegates aren't playing ball. Thanks to hacked emails showing Democratic Party operatives privately manoeuvring against Mr Sanders during the primaries and Mrs Clinton's selection of Virginia Senator Tim Kaine, a moderate, as her running mate, there's considerable anger among rank-and-file progressives in Philadelphia.
That frustration has already manifested itself in boos for outgoing Democratic Committee head Debbie Wasserman Schultz, protest marches, a contemplated walkout by some Sanders delegates during Mrs Clinton's Thursday night speech and possibly even the formal nomination of a more liberal alternative to Mr Kaine on Tuesday.
If any such events transpire, it could seriously undermine Mrs Clinton's efforts to defeat Donald Trump, who is already doing his best to foment Democratic discord.
Mr Sanders may join the pro-Clinton chorus, but as one Sanders delegate said on Monday, the movement he led is now bigger than him and, even if he falters, it will continue without him, party unity be damned.
Mr Sanders said that the departure of Ms Wasserman Schultz from the DNC would "open the doors of the party to people who want real change".
Mrs Clinton's campaign team has said the hackers who accessed the DNC emails were Russians who want Mr Trump to win the election.
One of the emails from a Democratic party official suggested raising Mr Sanders' faith as a means to discredit him. Other emails openly disparaged him and expressed a preference for his rival.
Sanders supporters have long claimed that the party's governing body, which was meant to remain neutral, had favoured the former secretary of state.
Before he takes the stage at the Wells Fargo Center, there will be speeches by First Lady Michelle Obama, liberal firebrand Elizabeth Warren and New Jersey senator Cory Booker.
About 5,000 party delegates are among the 50,000 people expected to attend the four-day convention, which will end on Thursday with Mrs Clinton formally accepting the nomination for president.