Hillary Clinton's presidential campaign has condemned the burning of an Israeli flag outside the Democratic National Convention.

An Israeli flag was set alight by what appeared to be Bernie Sanders supporters on Tuesday night as protesters chanted "Long live the intifada" outside the Wells Fargo Arena in Philadelphia.

Sarah Bard, the campaign's director of Jewish outreach, said in an email to the Israeli press on Wednesday: "Hillary Clinton has always stood against efforts to marginalise Israel and incitement, and she strongly condemns this kind of hatred. Burning the Israeli flag is a reckless act that undermines peace and our values."

Sanders supporters have reacted angrily to leaked emails apparently showing party officials had worked against him in favour of Clinton in a breach of their responsibility to be neutral in the primary contest. Thousands of activists supporting the Vermont senator have taken to the streets to protest, some wearing tape across their mouths, despite calls by Sanders for them to support Clinton.

"This was not a convention. This was a four-day Hillary party. And we weren't welcome," said Liz Maratea, a New Jersey delegate. "We were treated like lepers."

In the streets outside, protesters began scaling eight-foot-high walls blocking off the secure zone around the arena's car park, and several were detained by police.

Shortly after Sanders officially endorsed Clinton on Monday night, his campaign chief told the Times of Israel that one of his priorities was "fresh thinking" on the Israel-Palestine conflict.

He knows we'll have to work through this," Weaver said. "It's a very difficult situation over there, obviously. It's very complicated. And it's a situation we're going to have to work through."

You can watch the video of the flag-burning here.