Around 20,000 people marched in Madrid Saturday in support of Palestinians, a day after the UN's top court said Israel must prevent genocidal acts in its war with Hamas.

Many of the marchers carried banners and placards denouncing the "genocide" in Gaza, which has been under relentless bombardment and siege since the October 7 attack on Israel by Hamas.

Some carried Palestinian flags and shouted slogans denouncing Israel. Others had banners thanking South Africa for having brought the case against Israel to the International Court of Justice (ICJ).

South Africa accused Israel of breaching the 1948 UN Genocide Convention, set up after World War II and the Holocaust.

In its ruling on Friday, the ICJ said Israel must prevent genocide in its war with Hamas and allow aid into Gaza, but stopped short of calling for an end to the fighting.

The ruling was denounced by Israeli Prime Minister Benjamin Netanyahu as "outrageous" and while many countries welcomed the ruling, others, such as Britain expressed reservations.

Spain, one of the most critical voices in Europe of Israel's offensive against Hamas, was one of those to welcome Friday's ruling.

Relations between the two countries have soured over Madrid's position on the issue.

Israel recalled its top diplomat in Madrid in November after Spanish Prime Minister Pedro Sanchez expressed doubts about the legality of Israel's war in Gaza. She returned in January.

- 'Children are dying' -

"They have been without water, without food, without anything, for almost 110 days," one Madrid demonstrator, 54-year-old Lobna Elnakhala, said of the situation in Gaza.

"Children are dying and living in a very difficult situation."

Some banners called for sanctions to be levied against Israel.

The Madrid authorities put the turn-out at 20,000.

Israel's military campaign began soon after Hamas's October 7 attack that resulted in about 1,140 deaths in Israel, mostly civilians, according to an AFP tally of official figures.

Militants also seized about 250 hostages and Israel says around 132 of them remain in Gaza, including the bodies of at least 28 dead captives.

Israel has vowed to crush Hamas, and Hamas-ruled Gaza's health ministry says the Israeli military offensive has killed at least 26,257 people, most of them women and children