Thousands of protesters gathered in Washington on Saturday to demand a ceasefire in Gaza where thousands have been killed in an Israeli offensive since an attack by Palestinian Islamists Hamas, and to denounce President Joe Biden's policy towards the war.

Protesters carried placards with slogans such as "Palestinian Lives Matter," "Let Gaza Live" and "Their blood is in on your hands," as the U.S. government continued to reject demands to add its voice to calls for a blanket ceasefire.

Activists called the planned protest a "National March on Washington: Free Palestine" and organized buses to the U.S. capital from across the country for demonstrators to attend, said coalition group ANSWER, an acronym for "Act Now to Stop War and End Racism."

"What we want and what we demand is a ceasefire now," said Mahdi Bray, national director of the American Muslim Alliance.

The demonstration was among the largest pro-Palestinian gatherings in the United States and among the biggest for any cause in Washington in recent years.

Crowds began gathering at Freedom Plaza near the White House in the afternoon before the protest started with a moment of silence as demonstrators held up a large poster with names of Palestinians killed since Israel's massive retaliation began.

The deep-rooted Israeli-Palestinian conflict reignited on Oct. 7 when scores of fighters from Hamas, which controls the Gaza Strip, crossed into Israel, killing at least 1,400 people.

Israel has since struck Gaza from the air, imposed a siege and launched a ground assault, stirring global alarm at humanitarian conditions in the enclave. Gaza health officials said at least 9,488 Palestinians had been killed as of Saturday.

The growing number of civilian deaths has intensified international calls for a ceasefire, but Washington, like Israel, has so far dismissed them, saying a halt will give Hamas chance to regroup.

A group of independent United Nations experts has also called for a humanitarian ceasefire, saying time was running out for Palestinians there who are at "grave risk of genocide".

"Biden, Biden you cannot hide, you signed up for genocide," protesters chanted in Washington on Saturday.

Washington has sought to persuade Israel to accept localized pauses, which Israel has thus far rejected.