U.S. Democrats open their national convention Monday, set to affirm former U.S. Secretary of State Hillary Clinton as the party's 2016 presidential nominee, but embroiled in controversy over leaked emails that show how party leaders sought to ease her path to the nomination by mocking her challenger, Vermont Senator Bernie Sanders.
Debbie Wasserman Schultz, head of the Democratic National Committee, resigned from the post Sunday in the aftermath of the WikiLeaks disclosure of nearly 20,000 emails. She was jeered Monday by Sanders' supporters as she spoke to a group of delegates from her home state of Florida, where she is a congresswoman, but she did not talk about the emails.
Wasserman Schultz told the gathering she gladly accepted Clinton's offer to be a campaign surrogate for her, after leaving as party chief at the end of the four-day convention. She was set to gavel in the convention's opening session, but later was taken off the schedule.
WATCH: Bernie Sanders on Wasserman Schultz resignation
Sanders, a democratic socialist who waged a tough campaign against Clinton before she claimed the nomination, said he was outraged by the email disclosures that disparaged him, but said they justified his claims that party officials favored Clinton in her quest to become the country's first female president.
Sanders met with his supporters at the convention in the eastern city of Philadelphia, but heard boos and angry chants when he called for Clinton's election over her Republican opponent, real estate tycoon Donald Trump.
"Trump is a bully and a demagogue," Sanders said. "We have got to defeat Donald Trump."
Sanders is set to deliver a key address for Clinton her on Monday. First lady Michelle Obama and a progressive Democratic favorite, Massachusetts Senator Elizabeth Warren, also are set to make the case for Clinton.
The FBI said Monday it is probing what it described as a "cyber intrusion" at Democratic headquarters that resulted in the WikiLeaks disclosures.
Democratic officials say the emails were hacked from their official computers by "Russian state actors," a claim some U.S. computer experts say was possible, but a contention mocked by Trump.
"The new joke in town is that Russia leaked the disastrous (Democratic National Committee) emails, which never should have been written (stupid), because (Russian President Vladimir) Putin likes me," Trump said in a Twitter message.
Two new polls Monday showed Trump, a one-time television reality show host seeking his first elected office, edging ahead of Clinton in the days after the Republican convention.
Republican convention speakers pilloried Clinton's record for four days and delegates repeatedly shouted, "Lock her up!" at mentions of her handling of classified material on her private email server while she was the country's top diplomat from 2009 to 2013. The FBI recently concluded that she was "extremely careless" in dealing with the national security documents, but that no criminal charges were warranted.
U.S. presidential candidates typically get a bump in public support after their nominating conventions and Trump showed a sizable 10-point gain in a CNN/ORC poll. The news organization's survey shows Trump with a 48 to 45 percent lead, compared to its last poll showing Clinton with a 49-42 percent advantage.
A CBS News national poll showed Trump with a 44-43 percent lead.
More than 5,000 delegates are among the 50,000 people set to attend the gathering in Philadelphia, with some Sanders supporters continuing to protest Clinton's nomination.