Protesters against Israel's war in Gaza briefly breached the outer security fence of the Democratic convention in Chicago, hours before US President Joe Biden was set to pass the torch to new nominee Kamala Harris.

A small group of around 100 demonstrators broke off from a larger march involving thousands of people and targeted the metal barriers surrounding the United Center on the first day of the party gathering.

Police in blue helmets with shields and carrying black batons prevented them from getting to the inner cordon. One demonstrator clad in black was carried out by their arms and legs by several officers, an AFP correspondent saw.

Protest groups have called for mass demonstrations throughout the week against the Biden-Harris administration's support for Israel's war on Hamas following the Palestinian militant group's deadly October 7 attacks.

Chicago police said in a statement that protesters "breached a portion of anti-scale fencing along the Democratic National Convention's outer perimeter."

"Law enforcement personnel were immediately on-scene and contained the situation. At no point was the inner perimeter breached, and there was no threat to any protectees," police said.

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Police later advanced on a park near the convention center to clear it of demonstrators.

Chants of "Free Palestine" and "Let's March" continued as about half a dozen holdout activists, one wearing a pink gas mask, began to leave.

The Gaza war has been a hugely divisive issue for the Democratic Party ahead of the November 5 election.

It has threatened to alienate Muslim and Arab-American voters, once a reliable Democratic voting bloc, particularly in key battleground states.