"I know we're out, we're enjoying ourselves and we're trying to listen to some tunes at a festival... believe me lads, I wish I didn't have to do this," he said at the south London event.
"But the world's not listening. The world needs to see solidarity of 20,000 people in a park in London chanting, 'free free Palestine!'"
The chant echoed out around Brixton's Brockwell Park.
The UN said on Friday that Gaza was in the "cruellest phase" of war, with 9,000 trucks' worth of aid ready at the border for the Palestinian territory.
"Let's remember how lucky we are to be in a field with our friends and not being bombed from the sky," Kneecap's frontman, who goes by the stage name Mo Chara, told the audience on Friday night.
Israel launched a military campaign in Gaza in response to the Palestinian group Hamas's cross-border attack on 7 October 2023, in which about 1,200 people were killed and 251 others were taken hostage. Fifty-eight hostages remain in Gaza, and Israeli Prime Minister Benjamin Netanyahu said on Wednesday that 20 are believed to be alive.
At least 53,762 people, including 16,500 children, have been killed in Gaza since then, according to the territory's Hamas-run health ministry.
To their many fans, Kneecap are relatable, hedonistic provocateurs, mixing rapid-fire anti-establishment lyrics with danceable bass-heavy beats. Following in the footsteps of anti-establishment rap groups like NWA and Run The Jewels, the trio present themselves as dissident underdogs, giving a voice to the oppressed.
To their critics they are dangerous upstarts who have now gone too far.