Former Democratic Presidential hopeful Bernie Sanders drew loud "boos" from his delegates when he told the crowd of nearly 1,900 that "We have got to elect Hillary Clinton," a media report said.
The eruption came during a speech on Monday in which Sanders was cheered for nearly every point he made -- including his call to defeat the US Republican Presidential nominee Donald Trump, USA today reported.
"This is the real world that we live in," he said, following up his call to elect Clinton.
"Trump is a bully and a demagogue. Trump has made bigotry and hatred the cornerstone of his campaign," he added.
Monday's convention followed angry protests by Sanders supporters who already hit Philadelphia's streets to oppose Clinton's nomination. Sanders has endorsed Clinton and has said he will do whatever was necessary to defeat Trump.
Michael Tafe of Hingham said that he and other Sanders delegates continued to believe the Vermont Senator as the best candidate to defeat Trump and expressed unhappiness with Clinton's decision to choose Virginia Senator Tim Kaine as her running mate.
"We feel alienated by the Clinton campaign," Tafe said.
"They have made zero effort to reach out to us. Hillary's Vice Presidential choice is doubling down on her moderate platform and I think the people in this room are smart enough to realise once she gets into office, she's just going to flip," Tafe noted.
Monday's convention events were taking place as delegates absorb Sunday's announcement by Republican Debbie Wasserman Schultz of Florida that she would step down as chairwoman of the Democratic National Committee after the convention over leaked emails that revealed earlier DNC attempts to undermine Sanders' Presidential campaign, USA today reported.
Democrats had hoped to project an image of unity at the convention, but the emails make that more difficult. So does Clinton's appointment of Wasserman Schultz as honourary chairwoman of her campaign's 50-state program to elect Democrats.
"It kind of all reaffirms in the minds of Bernie delegates that we were given a raw deal, that we are dealing with a rigged system, that the primary was very much rigged from the very beginning in favour of Clinton," said Karen Bernal, one of the leaders of the California Sanders delegation.
A majority of Sanders delegates surveyed in a straw poll wanted to protest the nominations of both Clinton and Kaine on the floor, said Norman Solomon, a Sanders delegate from California and national coordinator of the independent "Bernie Delegates Network."
"If Hillary Clinton wants to move today towards more party unity, she certainly has it within her power to say it's a mistake to make an honourary chair out of Debbie Wasserman Schultz," Solomon said, adding, "We shouldn't be honouring someone who ran such a, we now know, disreputable shop at the DNC."
Sanders issued a statement saying that Wasserman Schultz had made the right decision for the future of the Democratic Party.
"While she deserves thanks for her years of service, the party now needs new leadership that will open the doors of the party and welcome in working people and young people," Sanders said, adding, "the party leadership must also always remain impartial in the presidential nominating process, something which did not occur in the 2016 race."
Sanders had again called for Wasserman Schultz's resignation but said Democrats should focus on defeating Trump, whom he called "perhaps the worst Republican candidate that I have seen in my lifetime."
"We have to elect Secretary Clinton, who on every single issue -- fighting for the middle class, on health care, on climate change -- is a far, far superior candidate to Trump," Sanders said.
He will note the "most progressive platform in Democratic Party history" includes agreements he reached with Clinton to expand access to health care and make public college tuition-free for students from families with annual incomes up to $125,000 a year.
Sanders also will tell the 13 million voters who supported him during the primary season that the political revolution they helped him launch continues.
Unlike Senator Ted Cruz of Texas, who shocked the Republican Party's national convention in Cleveland last week by not endorsing Trump during his speech, Sanders endorsed Clinton at a New Hampshire rally on July 12 and said he will do everything he can to help her defeat Trump.
Sanders will address the Democratic convention the same night that first lady Michelle Obama and immigration activist Astrid Silva will speak. Convention officials said the night's "United Together" theme will focus on building an economy that works for everyone, "not just those at the top"—one of the mantras of Sanders' campaign, USA Today reported.