Arguing it was acting to defend itself and was fighting Hamas, not the Palestinian population, Israel called on the International Court of Justice (ICJ) to dismiss the case as groundless and reject South Africa's request to halt the offensive.
"This is no genocide," lawyer Malcolm Shaw said.
Israel launched its war in Gaza after a cross-border rampage on 7 October by militants from Hamas, which is sworn to Israel's destruction. Israeli officials said 1,200 people were killed, mainly civilians, and 240 taken hostage.
"The appalling suffering of civilians, both Israeli and Palestinian, is first and foremost the result of Hamas' strategy," the Israeli foreign ministry's legal adviser, Tal Becker told the court.
"If there were acts of genocide, they have been perpetrated against Israel," Becker said. "Hamas seeks genocide against Israel," he added.
South Africa asked the court on Thursday to impose emergency measures ordering Israel to immediately halt the offensive.
It said Israel's aerial and ground offensive - which has laid waste to much of the enclave and killed almost 24,000 people, according to Gaza health authorities - aimed to bring about "the destruction of the population" of Gaza.
Israel rejected the accusations, saying it respected international law and had a right to defend itself.
"When the cannons roar in Gaza the law is not silent," Deputy Attorney General Gilad Noam told the court.
The 1948 Genocide Convention, enacted in the wake of the mass murder of Jews in the Nazi Holocaust, defines genocide as "acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group".
Israel, its defence team argued, was doing what it could to alleviate the humanitarian suffering in Gaza, including efforts to urge Palestinians to evacuate.
The court is expected to rule on possible emergency measures later this month but will not rule at that time on the genocide accusations. Those proceedings could take years. The ICJ's decisions are final and without appeal, but the court has no way to enforce them.
Palestinian backers with flags marched through The Hague and watched proceedings on a giant screen in front of the Peace Palace. As the Israeli delegation spoke in court, they chanted: "Liar! Liar!"
Asked what she thought of Israel's arguments that the Gaza campaign was a matter of self-defence, Neen Haijjawi, a Palestinian who recently came to Netherlands said: "How can an occupier that's been oppressing people for 75 years say it's self-defence?"
Israeli supporters were holding a separate gathering of family members of hostages taken by Hamas.
Israel has said South Africa is acting as a mouthpiece for Islamist Hamas, which is designated as a terrorist group by the United States, the European Union, Britain and several other nations. South Africa has rejected that accusation.
Since Israeli forces started their offensive, nearly all of Gaza's 2.3 million people have been driven from their homes at least once, leading to a humanitarian catastrophe.
Post-apartheid South Africa has long advocated the Palestinian cause, a relationship forged when the African National Congress' struggle against white-minority rule was supported by Yasser Arafat's Palestine Liberation Organisation.
"My grandfather always regarded the Palestinian struggle as the greatest moral issue of our time," Mandla Mandela, a grandson of the late South Africa president Nelson Mandela, said at a rally in support of the Palestinians in Cape Town.
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