PHILADELPHIA — Sen. Bernie Sanders spoke to every one of his nearly 1,900 delegates Monday afternoon, beseeching them to join him in a united Democratic front for the November election. In an address that had been closed to press, but was partially opened (and widely live-streamed by attendees), Sanders went through every item of his platform, then made an awkward pivot.

"Immediately, right now, we have got to defeat Donald Trump," said Sanders, to cheers.

"And we've got to elect Hillary Clinton," Sanders added.

Sen. Bernie Sanders addresses his supporters and delegates at the Philadelphia Convention Center during the first day of the Democratic National Convention on July 25 in Philadelphia. (Ricky Carioti/The Washington Post)

The boos in the Philadelphia Convention Center were overwhelming and, sustained for 30 seconds, as Sanders stood tight-lipped on stage. There were chants of "We Want Bernie!" and "Green Party" and "take it to the floor" — an apparent call for Sanders delegates to be allowed to cast their votes, and not be released to vote for Clinton.

Sanders, who endorsed Clinton two weeks ago, seemed taken aback by the level of anger. "This is the world we live in," he explained. His wife, Jane O'Meara Sanders, could be heard speculating that the booing delegates did not realize that there would be a roll call vote.

"Trump is a bully and a demagogue," said Sanders, re-opening the playbook he used in the final weeks of the Democratic primary. "He does not respect African Americans and Latinos."

"So has she! So has she!" yelled one heckler, referring to Clinton. There was another eruption of booing, culminating in a louder round of "We Want Bernie!" chants.

"I will do everything I can to see that he is defeated," said Sanders, ending the Clinton part of the speech.

Before the booing began, the Sanders delegate meeting was busy, raucous, and joyful. Having given up his Secret Service protection with the end of his campaign, Sanders was guarded only by some convention security, allowing delegates to crowd around the stage where he'd speak. And he did not release his delegates — setting up, for the first time since 1992, a roll call vote where opponents of the presumptive nominee will get their votes counted en masse.

That encouraged some of the diehard Sanders voters who desperately wanted to avoid a vote for Clinton. Earlier today, members of the Bernie Delegates Network -- a new progressive coalition of around 1250 of Sanders's 1900 delegates -- told reporters that delegates who lived in deep red or deep blue states were free to vote for third party candidates. The cries of "Green Party" in the convention rooms were references to Jill Stein, that party's likely nominee, who has make several public appearances per day in Philadelphia to ask for Sanders supporters to cross over.

Some Sanders delegates were terrified of that mood gripping their peers. "I don't want a third party, not after what happened in my state in 2000," said Samantha Herring, 46, a Sanders delegate from Florida's panhandle who toted a sign with the popular slogan Love Trumps Hate. "I'm a Democrat first, and a lot of things that have transpired here have been very undemocratic. But I assure you, if Bernie says 'get to work,' I'll get to work."

That work, she said, would start after the convention. Until Thursday, at least, she was voting for Bernie Sanders. At the mid-day rally, even some Sanders surrogates who had endorsed Clinton, like former NAACP president Benjamin Jealous, avoided mentions of her name -- and thereby avoided hecklers.