The march, held peacefully under constant rain along Copacabana Beach, was watched by dozens of police and soldiers deployed as security for the summit to be held Monday and Tuesday.
The meeting will see heads of state and government, including US President Joe Biden and Chinese President Xi Jinping, discuss coordination on international issues.
The Rio protesters, a few wearing Arabic keffiyeh scarves, held aloft the Palestinian flag and banners, including one reading "Break Brazil-Israel Relations" and demands that Israeli allies stop financing its military offensives in Gaza and in Lebanon.
"We're here to make a contrast with the G20 summit," said Tania Arantes, 60, from one of the Brazilian unions organizing the protest.
She said the march embraced a number of other leftist issues too, such as climate change, the fight against poverty and a demand to tax the super-rich, because the leaders at the summit "have economic control over nations they believe are subordinate in this globalized world."
One marcher, Giancarlo Pereira, a 43-year-old veterinarian, said the multiple leftist issues converged with the Palestinian cause "because the big companies fueling the war (being conducted by Israel in Gaza) are the billionaires of the world."
A short distance along Copacabana Beach, another protest was being staged with activists placing rows of plates with red crosses on them in the sand.
The 733 plates laid out represented the 733 million people in the world the UN says suffered from hunger last year.
Another demonstration was to take place in Rio later Saturday organsed by a Brazilian Indigenous umbrella group, the Articulation of Indigenous People of Brazil, to underline a perceived lack of effort by rich countries to combat climate change.
The various protests were taking place as activists, NGOs and civil society bodies took part in a pre-summit G20 Social event in Rio promoted by Brazilian President Luiz Inacio Lula da Silva.
Lula was to receive a list of action points drawn up by that event to help inform summit discussions on Monday and Tuesday.
Headline talks to take place at the summit include an initiative by Lula for a Global Alliance against Hunger and Poverty, and international efforts to mitigate global warming.
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