Marchers, backed by more than 250 community organisations, unions and prominent public figures turned up at rallies in more than 40 cities on Sunday to demand sanctions and an end to Australia's arms trade with Israel.
"The movement is the biggest it's ever been," Palestine Action Group's Sydney spokesman Josh Lees told AAP.
Organisers anticipate up to half a million people will turn out by the day's end, backing up the estimated 300,000 that attended the Sydney Harbour Bridge march in early August, which they say was a reflection of the huge outpouring of support for Palestinians.
"That bridge march has generated so much momentum around the country," Mr Lees said.
"The dam has burst in terms of support for Palestine and opposition to this genocide."
The action comes days after the United Nations confirmed famine in parts of the Gaza Strip for the first time, as Israel prepares for a military takeover of the entire city.
"It adds even more to the urgency of why we need to mobilise to demand our government takes real action to stop this," Mr Lees said.
More than 60,000 Palestinians have been killed including 18,000 children since October 7, 2023, when Hamas killed 1200 people and took more than 200 hostages at an Israeli music festival, according to the United Nations.
The push for change comes after a magistrate vetoed plans by organisers to march across Brisbane's Story Bridge, a move welcomed by Queensland Police.
At least 7000 gathered at Queens Gardens, with speeches kicking off just after 1pm.
A larger turnout congregated just before midday outside Victoria's State Library in Melbourne, chanting "sanction Israel now" as a news chopper hovered above.
As protesters prepared to march to Flinders Street Station just before 1.30pm, organiser Nour Salman estimated the crowd to be about 30,000.
She said Australia's plans to recognise Palestinian statehood, along with like-minded allies at the next United Nations meeting in September, must be accompanied by tougher sanctions on Israel.
"Enough is enough. There is no ifs, buts or maybes," the Palestinian activist told reporters.
Greens Senator Mehreen Faruqi was critical of the late forced protest route change in Brisbane, declaring state and federal governments should be ashamed of themselves.
"The lengths the Queensland government is going through to stop people marching on the Story Bridge, frankly these scenes belong in a dystopian world," she told the Melbourne crowd.
"They belong in authoritarian regimes, they probably belong in Trump's America."
Independent Senator Lidia Thorpe called for Australians to boycott Israel in all its forms, while the leader of Victoria's union movement demanded the federal government stop trading parts used for F-35 fighter jets.
"We need Israeli officials to be arrested and face consequences," Victorian Trades Hall Council secretary Luke Hilakari said.
It was the 97th consecutive weekend of rallies for Gaza in Melbourne.
Protests are simultaneously taking place in Canberra, Adelaide, Perth and Darwin, as well as in regional cities, including Warrnambool and Geelong.
Israel has rejected criticism that its actions in Gaza amount to genocide, a claim that has also been brought against it before the International Court of Justice.
Prime Minister Benjamin Netanyahu has called the famine declaration an "outright lie".
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