Bernie Sanders has urged his supporters to back Democratic rival Hillary Clinton in her White House bid, drawing jeers and shouts of "We want Bernie" in a show of discord as the party kicked off its national convention.

The boos underscored the deeply felt anger his convention delegates feel at both Clinton's win and embarrassing emails leaked on Friday suggesting the party leadership had worked to sabotage Sanders' campaign for the nomination.

Debbie Wasserman Schultz resigned as party chairwoman on Sunday, a day before the start of the Philadelphia convention to formally nominate Clinton for the November 8 election, and on Monday she bowed to pressure and agreed not to open the convention.

Speaking to his supporters, Sanders savaged Republican presidential candidate Donald Trump, calling him a danger to the future of the country who "must be defeated".

But some in the room booed when he said "we have got to elect Hillary Clinton" and her vice presidential running mate, US Senator Tim Kaine of Virginia.

Sanders, a US senator from Vermont, tried to calm them down. "Brothers and sisters, this is the real world that we live in," he said, adding, "Trump is a bully and a demagogue."
Members of the crowd started screaming back: "So is Hillary."

"She stole the election!" someone else shouted.

The leak on Friday of more than 19,000 Democratic National Committee emails by the WikiLeaks website put the spotlight back on Sanders' failed bid to win the Democratic nomination, and in particular on his complaints during the campaign that the party establishment was working to undermine him.

A democratic socialist, Sanders ran an unexpectedly tough race against former Secretary of State Clinton, galvanising young and liberal voters with his calls to rein in Wall Street and eradicate income inequality.

On Monday, Sanders brought the loudest cheers when he noted that Wasserman Schultz, a US senator from Florida, had resigned as the Democratic National Committee chairman over the email controversy.

"Her resignation opens up the possibility of new leadership at the top of the Democratic Party," Sanders said, adding that the leadership should be made up of "people who want real change."

Wasserman Schultz's resignation is effective at the end of the convention, but on Monday, she said she would not speak as planned at the opening of the convention on Monday afternoon.

"I have decided that in the interest of making sure that we can start the Democratic convention on a high note that I am not going to gavel in the convention," she said.

On Monday morning Wasserman Schultz struggled to be heard above boos as she spoke to the Democratic delegation from her home state. Some protesters held up signs that read "Bernie" and "E-MAILS" and shouted "Shame," as she spoke.

The cache of leaked emails disclosed that DNC officials explored ways to undercut Sanders' insurgent presidential campaign, including raising questions about whether Sanders, who is Jewish, was an atheist.

The Clinton camp questioned whether Russians may have had a hand in the hack attack on the party's emails in an effort to help Trump, who has exchanged words of praise with Russian President Vladimir Putin.

The FBI said on Monday it would investigate the nature and scope of the hack.

After lagging Clinton in most national polls for months, a CNN/ORC opinion poll on Monday gave Trump a three-point lead, 48 per cent to her 45 per cent.