For supporters of both Israel and Palestine, the wounds run deep after decades of bitter conflict.
Sydney, London, and Paris were among the dozens of global cities where anguished crowds took to the streets in solidarity.
Hundreds of Palestine supporters marched through Sydney in protest against the Opera House being lit up in blue and white in support of Israel following attacks by militant group Hamas.
Prime Minister Anthony Albanese had earlier called for the march to be called off out of respect for the loss of life in Israel.
"The actions of Hamas in this is completely indefensible," Albanese told 2GB radio, adding that "this was people killing innocent civilians".
NSW Greens MP Jenny Leong criticised the prime minister's comments and the lighting of the Opera House "in support of those bombing Palestinian people in Gaza into oblivion".
There was a heavy police presence keeping an eye on the Sydney march, with both mounted officers and some on foot.
The group organising the march, Palestine Action Group Sydney, said it would be a "mass, peaceful protest" and police also described the event as "peaceful".
A man who attended the march with an Israeli flag was arrested and removed "to prevent a breach of the peace", police said.
He was issued with a move-on order "for his own safety" but wasn't facing any further action.
Some estimates put crowd numbers at about 1000.
The Sydney Oprah House lit up in the colours of the Israel flag on Monday night.
Sydney's Jewish community said police had warned them to stay away from the Opera House while it was lit up.
Flinders Street Station in Melbourne was also lit up blue and white in solidarity with Israel.
In Paris, the Eiffel Tower was illuminated with the colours of Israel.
Kept apart by police and barriers, supporters gather at the Israeli Consulate in New York.
In Argentina, people wave Palestinian flags during a demonstration near the Israeli Embassy in Buenos Aires.
In Pakistan, boys step on a representation of the US and Israeli flags to protest against Israeli airstrikes on Gaza and to show solidarity with Palestinian people in Karachi.
People wave flags and flares as they take part in a demonstration in support of Palestine at the Israeli Embassy in London.
Here, protesters are demanding that Israel put an end to its cycle of violence against the Palestinian people.
The call to action comes after the Israel Defence Forces initiated air strikes on Gaza late on Saturday in response to a surprise attack from the Palestinian militant group Hamas.
A protester holds a flare during a pro-Palestinian demonstration in London.
Earlier that day in London, protesters shout during the Jewish Community Vigil for Israel.
Where protests appear, counter-protesters from the other side arrive to stand their ground.
Pro-Israel protesters wave signs during a pro-Palestinian demonstration outside the Israeli embassy in San Francisco.
A pro-Israeli counter-protester waves a flag during a march of Palestinian supporters in Toronto, Canada.
Palestinian supporters protest during a march in Toronto.
People hold Colombian and Israeli flags to show support for Israel and protest the war outside the Israeli embassy in Bogota, Colombia.