Bernie Sanders, Hillary Clinton's former rival, took the stage at the Democratic convention Monday night, where he was greeted by massive applause as he thanked the Americans who supported his campaign and voted for the "political revolution" he advocated for in the lengthy primary.

The senator from Vermont, who spoke at the end of the night, received huge cheers and chants from the crowd in Philadelphia, as supporters chanted and waved "Bernie" signs when he stepped on to the stage.

"Together, my friends, we have begun a political revolution to transform America and that revolution, our revolution, continues," he told the crowd.

Sanders said the election wasn't about his own candidacy, or that of Clinton or Republican rival Donald Trump.

"This election is about, and must be about, the needs of the American people and the kind of future we create for our children and our grandchildren," Sanders said.

He said the U.S. needs leadership that brings people together and makes the country stronger, not leadership that insults and seeks to divide people.

"By these measures, any objective observer will conclude that based on her ideas and her leadership Hillary Clinton must become the next president of the United States."

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Sanders's remarks drew a mixed reaction from the crowd, with some cheering and others chanting "Bernie!" as he spoke.

He revisited education issues, health-care and the justice system. He said it's "no secret" that he and Clinton disagree on a number of issues, but said the two teams came together to produce what he called "the most progressive platform in the history of the Democratic Party."

'No secret' Clinton and Sanders have their differences0:46

Trump reacted to Sanders's speech on Twitter in a series of posts in which he said Republicans "welcome all voters who want a better future for our workers."

Sanders was blunt and brief in his reply, which was reminiscent of Clinton's entreaty to Trump last month to "delete your account." 

Sanders appeared after speeches by Senator Elizabeth Warren and Michelle Obama, who used her time on stage to look back at her time in Washington and look forward to the next election, throwing her support behind Clinton.

'I'm with her'

Obama, who has been in Washington since her husband was first elected president in 2008, talked about Clinton's record advocating for children and her time as secretary of state. She praised Clinton and said the next president needs to be "steady, measured and well-informed."

Michelle Obama endorses Hillary Clinton at DNC2:06

"What I admire most about Hillary is that she never buckles under pressure," Obama said. "She never takes the easy way out and Hillary Clinton has never quit on anything in her life."

Obama said she was in Philadelphia speaking because of what Clinton stands for and the type of president she could be.

"In this election, I'm with her."

Thanks to Clinton, Obama said, sons and daughters in the U.S. now take for granted that a woman can be president. 

She urged people to get out and support Clinton, saying Democrats can't afford to be "tired or frustrated or cynical" ahead of the general election against Trump.

Democratic convention

The four-day Democratic National Convention kicked off on Monday. (Drew Angerer/Getty Images)

Obama also took aim at Trump's campaign slogan.

"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."

Warren, a senator from Massachusetts, took the stage after Obama, where she thanked Sanders for reminding "us what Democrats fight for every day." Warren praised Clinton and slammed Trump, saying the Republican nominee was peddling a vision of a country built on fear.

Turbulent start to convention

The first night of the four-day event was marked by obvious signs of dissent and disagreement within the party. As speaker after speaker took the stage, chants of "Bernie" echoed, and boos could be heard nearly every time Clinton's name was raised.

Chants against the Trans-Pacific Partnership (TPP) trade deal spread through the arena several times, as Democratic speakers tried to make the case for Clinton.

Senator Al Franken roasted Trump in one appearance, and was on stage again with comedian Sarah Silverman not long after to introduce singer-songwriter Paul Simon. Silverman, who had been an outspoken Sanders supporter, said Monday she would proudly vote for Clinton.

"Can I just say to the 'Bernie or Bust' people —  you're being ridiculous," Silverman said at one point. 

Sarah Silverman: 'You're being ridiculous'0:19

Earlier Monday, several hundred Sanders backers marched down Philadelphia's sweltering streets outside the arena. Signs carried messages such as "Never Hillary" and even in one case "Just go to jail, Hillary," an echo of the Republicans' "Lock her up."

Much of Monday's program appeared aimed at giving Sanders's backers an opportunity to express their frustration before the convention moved on to focus on speakers who strongly support Clinton.

Monday also featured some non-politicians, as speakers came out to talk about addiction issues and mental health.

Singer-songwriter Demi Lovato talked about living with mental illness and her experience getting help.

"I'm proud to support a presidential candidate who will fight to ensure all people living with mental health conditions get the care they need to lead fulfilling lives — that candidate is Hillary Clinton," Lovato said to cheers. "Let's make her the next president of the United States."

Demi Lovato at the DNC1:57

Senator Bob Casey, a Democrat from Pennsylvania, urged people to support Clinton, calling her a "leader with a proven track record" of fighting for an economy that works for everybody.

"The man who wants to make America great doesn't make anything in America," Casey said in reference to Trump, as the night's speeches turned to the economy ahead of the main speakers of the night.

Protests, chants

Sanders had already scored a major victory with the forced resignation of party chair Debbie Wasserman Schultz following the release of emails showing her staff favoured Clinton during the primary despite vows of neutrality. But Sanders's aides reached out to the Clinton campaign Monday afternoon to express concerns that the chairwoman's ouster wouldn't be enough to keep supporters from disrupting the convention, according to a Democratic official.

Silenced

A delegate from Michigan registers dissent on the first day of the Democratic National Convention in Philadelphia. (Chip Somodevilla/Getty Images)

The 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.

Michigan delegate Bruce Fealk said he understood Sanders's position and why the Vermont senator made the request. But Fealk also said: "I'm really annoyed.… I haven't decided yet. I want to support Bernie, but I also want to voice my displeasure with the Democratic Party."  

DNC apologizes to Sanders, supporters

Moments after the convention opened, the DNC also apologized to Sanders and his backers "for the inexcusable remarks made over email." The statement was signed by DNC leaders, though Wasserman Schultz's name was notably absent.

The Florida congresswoman's resignation is effective later this week, though she also stepped down from her official convention duties. The mere sight of her on stage had been expected to prompt strong opposition from Sanders's backers.

Sanders 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 that wields significant power in the primary process.

With files from CBC's Jennifer Walter and The Associated Press