Thousands of pro-Palestine protesters marched in Washington, London and elsewhere on Saturday as part of a "global day of action" to demand an immediate cease-fire in the Gaza fighting and to oppose US and British support for Israel.

In Washington, large crowds waved Palestinian flags as the mostly young protesters -- many wearing the traditional keffiyeh -- gathered in a show of solidarity on the 99th day of the Israel-Hamas war in the Gaza Strip.

"Cease-fire now," people chanted, while carrying banners and posters that read "Free Palestine" and "End the War on Gaza."

On a stage a few blocks from the White House, several Palestinian-Americans -- originally from Gaza, but now living in US states from Michigan to Texas -- offered emotional accounts of friends and relatives killed or wounded in Gaza.

They exhorted US President Joe Biden to end military and financial support for Israel.

"President Biden can easily stop this madness" by pressuring Prime Minister Benjamin Netanyahu of Israel, one speaker said to wide applause.

London, meantime, saw its seventh pro-Palestinian demonstration since October 7, when Hamas militants invaded southern Israel and killed around 1,140 people, mostly civilians, according to an AFP tally based on official figures.

Israel vowed to destroy Hamas and launched a relentless bombardment of Gaza that has killed at least 23,843 people, mostly women and children, according to the latest toll from the territory's health ministry.

Around 1,700 police were on duty Saturday to ensure security for the London protest.

"We want to show the people of Palestine we are with them, and to speak up against our government as well," 27-year-old health service worker Maleeha Ahmed, who was at the march with her family, told AFP.

"They are playing a very, very big role in allowing Israel to continue what they are doing and it's just not acceptable," she said.

Another marcher, Dipesh Kothar, 37, said it was "very frustrating to sit and watch the world do nothing."

"That's why we come out to show support for the Palestinian people and show our unhappiness with the governments around the world," he said.

Saturday's marches held particular significance given US and British air strikes in Yemen this week against Huthi bases, after the Iran-backed militants attacked ships in the Red Sea in solidarity with Gaza.

The day of action, called by a British organizing coalition, involved protests in 30 countries.

Kate Hudson of the Campaign for Nuclear Disarmament, which is part of the coalition, said the event was "to demand a permanent cease-fire and a lasting political settlement for all Palestinians."

She said the British government "must end its support for Israel's brutal war in Gaza, and join the wider international community in condemning its war crimes."