Die-hard Bernie Sanders supporters have descended on Philadelphia for this week's Democratic National Convention, many so irate with party flagbearer Hillary Clinton that they are prepared to contemplate the once-inconceivable alternative: President Donald Trump.

Key points: Sanders supporters are angry at revelations some DNC staff sought to undermine him

Sanders supporters are angry at revelations some DNC staff sought to undermine him Some say they will find it hard to vote for Hillary Clinton in November

Some say they will find it hard to vote for Hillary Clinton in November Bernie Sanders has endorsed Mrs Clinton and will address the party convention

Most supporters of Mr Sanders, who entered the race as an avuncular fringe candidate before gaining vast popular support, consider themselves on the opposite side of the political spectrum from the brash billionaire real estate tycoon who secured the Republican presidential nomination last week.

But several among the Sanders faithful who spoke with AFP during colourful street protests Sunday in Philadelphia offered no love for the candidate who will emerge this week as the first female presidential nominee of any major US party.

Some let it be known that they would prefer anyone — even Mr Trump — as president rather than see Clinton gain the White House.

Some Sanders fans say they'd accept a Trump presidency if it meant Mrs Clinton didn't win. ( AFP: Nicholas Kamm )

"The system, as Trump says, is rigged," said Shana Lin, a housewife from Virginia Beach who held a sign that read "You lost me at Hillary" as fellow "Berners" flooded the streets in the shadow of City Hall.

"Actually, at this point, I think Trump is a marginal step up, because (Clinton) has proven to lie to the American people over and over," she added.

"She lies, she's a warmonger, she's divisive. She is only out for herself."

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Several thousand protesters, many of them ardent Sanders backers, held demonstrations in Philadelphia far larger than any seen in Cleveland during the Republican convention.

It was clear the anger about Mrs Clinton winning the nomination has not abated, with Sanders supporters arguing Mrs Clinton's corralling of so-called superdelegates — party grandees who can vote for whomever they wish at the convention — gave her unfair advantage.

"The democratic process was stolen from us," steamed Steffanie Greer, 42, from Brooklyn.

A Bernie Sanders bus downtown before the Democratic convention in Philadelphia. ( AFP: Spencer Platt )

Email leak angers Bernie fans

The latest Clinton scandal — the leak of internal emails that signalled senior Democratic National Committee staff were trying to undermine the Sanders campaign — was the last straw for some.

Erupting on the eve of the convention, it highlighted divisions with the party at the moment Mrs Clinton was aiming to shore up unity in front of a national audience.

"It just proved what we have suspected all along, that this primary has been rigged against Bernie by the DNC," Sanders delegate Lauren Steiner said.

Mr Sanders himself has endorsed his former bitter rival in the primaries, and will likely reiterate that message Monday when he addresses the convention.

But despite their candidate now backing Clinton, many say they will not hold their nose and pull the lever in November for someone they loathe.

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"I'll never vote for Hillary, I'll vote for Jill Stein," said John Delahanty, 66, referring to the Green Party presumptive nominee who has openly encouraged Sanders himself to join the Green movement.

Would those in Bernie's camp be willing to endure a Trump presidency in order to stop Clinton?

Mr Trump in the White House would "be so crazy," acknowledged Sanders supporter Marie Adams, 66, of Boulder, Colorado.

But "I'll live with whatever for the next four years ... even if it's Trump."

While he is an untested political quantity, "the Clinton's have been nothing but bad," Adams said, noting Bill Clinton signed the North American Free Trade Agreement and a widely criticised crime bill.

Activists were angered by revelations some DNC staff were trying to undermine Mr Sanders. ( AFP: Jeff J Jackson )

AFP