Protesters against Israel's war in Gaza briefly breached the outer security fence of the he Democratic National Convention (DNC) in Chicago Monday, hours before US President Joe Biden passed the torch to new nominee Kamala Harris.

Biden, 81, did not directly address the security lapse in his hour-long speech, but he said "those protesters out in the street, they have a point. A lot of innocent people are being killed, on both sides" of the Israel-Hamas conflict.

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 Centre on the first day of the Democratic Party's 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 in Gaza following Hamas'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.

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.

Biden, in his speech, said his administration will keep working to "bring peace and security to the Middle East."

"We're working around the clock... to prevent a wider war and reunite hostages with their families, and surge humanitarian health and food assistance into Gaza now, to end the civilian suffering of the Palestinian people, and finally, finally, finally deliver a ceasefire and end this war."