Berlin (AFP) - Hillary Clinton on Monday denounced President Donald Trump as a "danger to democracy" and defended her scathing comments about former rival and Democratic primary frontrunner Bernie Sanders.

Clinton also warned that Russia would once again try to influence the presidential election in 2020, and said she would back any candidate taking on Trump.

"I will support the nominee of our party because I think that our current president is a clear and present danger to democracy and to our future," she told journalists at the Berlin film festival, where a new documentary about her life was being screened.

Yet she refused to endorse any specific candidate, saying she would "leave that to the voters".

The documentary has grabbed headlines for caustic remarks Clinton makes about Sanders, who took pole position in this year's race with a key win in Nevada on Sunday.

"Nobody likes him, nobody wants to work with him, he got nothing done. He was a career politician," she says in the four-part film.

She defended the comments on Monday, saying: "Politics is a contact sport, people say all sorts of things. They've certainly said all sorts of things about me."

- 'Sad dreamworld' -

Following revelations from US officials this week that Russia was once again meddling in the US election, Clinton called on voters to be "vigilant" against disinformation.

"We know disinformation is already a problem... the (US) intelligence community reported that the Russians are back in our cyber systems trying to influence the outcome of the primary and still trying to support Trump," she said.

"There is no debate. Anyone who tries to deny it is living in a dreamworld, and it's a sad dreamworld because we are letting our defences down."

Foreign powers, she said, would try "more and more" to influence elections in the US and Europe.

"If we don't protect our democracy then we are slowly but steadily surrendering it and losing control of it."

The former US secretary of state, who beat Sanders in the primaries but ultimately lost to Trump at the 2016 US presidential election, said the new documentary would tell "the real story" of her political career.

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"It shows the story of how hard political change is. That is an important story, because in a democracy, if you get discouraged and you give up on it, that leads to voices of extremism promising all sorts of things."

She claimed it showed her political successes in the face of "tremendous opposition, including being burned in effigy and being threatened".

Clinton remains a divisive political figure.

Even on the red carpet in a leafy, upmarket area of Berlin, her supporters begged her for autographs while a few isolated protesters chanted slogans such as "Hillary to hell!"