PHILADEPHIA, United States -- The first day of the Democratic convention included an FBI investigation, senior party figures getting booed and fears of a Russian plot to defeat Hillary Clinton.

If indeed Vladimir Putin's Kremlin intended to sow chaos in this U.S. presidential campaign, that mission was surely accomplished Monday.

The FBI announced it would investigate the hacking of the party's servers as thousands of incendiary emails were dumped onto the site Wikileaks just before Democrats gathered in Philadelphia.

The emails revealed mockery and opposition to insurgent candidate Sen. Bernie Sanders within the party's upper echelons. Their public release poured gasoline over embers of bitterness still smoldering from a long primary season.

The result: Party chair Debbie Wasserman Schultz announced she would step down after this week, while reports surfaced that the Clinton campaign would rather she disappeared sooner. She was drowned out by boos Monday while speaking to delegates from her home state. Party titan Nancy Pelosi, who endorsed Clinton, was also booed by attendees from her home state.

The good news for Clinton is that her popular rival appears determined to limit the damage.

Sanders was scheduled to speak to the convention Monday. He has repeatedly changed the subject when asked about the leaks, and said he's now focused on helping avert what he calls the disastrous prospect of a Donald Trump presidency.

"He is coming to double down on (his) endorsement (of her tonight)," said Clinton's campaign manager Robby Mook, who promised a starkly different convention than last week's Republican one, in several respects.

Mook said the speeches would be optimistic, offering ideas for helping people and showcasing Clinton's lifelong commitment to public service. He contrasted that with Trump, whose convention featured angry speeches, few solutions and a headliner whose lifetime cause, Mook said, has been his own personal self-aggrandizement.

He also said the party leadership is united behind Clinton, unlike the Republicans whose most famous members mostly stayed home or failed to endorse Trump.

"Our convention could not be a greater contrast."

Sanders speaks Monday night, as do First Lady Michelle Obama and senators Elizabeth Warren and Cory Booker. President Barack Obama, former president Bill Clinton and reportedly ex-New York mayor Michael Bloomberg will address the convention later in the week.

It's true that opposition to Trump persists in Republican ranks, but it was quiet in Cleveland. In Philadelphia, it's been noisy in the street. It remains to be seen whether that noise will spill into the main convention hall.

Many Sanders supporters are convinced the nomination was stolen from him. Some say they're still hoping for some miraculous, mathematically improbable come-from-behind win at the convention.

This is despite Clinton getting 25 per cent more votes in the primaries, and winning almost a dozen more contests. The party establishment also favoured her -- a fact the leaked documents make clear.

Polls say she's struggling to win over about one-third of Sanders supporters.

These devastating timed leaks have complicated the task.

As for the culprit, the security company hired by the Democrats to investigate announced last month that the breach was conducted by two well-known hacking operations linked to Russia's state security apparatus.

Trump has business ties to Putin allies; his campaign manager spent years working with the pro-Putin party in Ukraine; his top military adviser attended a party for the Kremlin news outlet Russia Today; and his policies critical of NATO allies and the European Union are in sync with those of the Russian leader.

The Clinton campaign says it fears the Russians may be trying to elect Trump.

"It's up to Donald Trump to decide what he thinks of it," Mook said of the possibility that Putin is playing havoc with a U.S. election.

"Voters need to think of it. It's troubling if it's true."