Rally organiser Palestine Action Group intends to draw attention to what the United Nations has described as worsening famine conditions in Gaza.
Organisers expect tens of thousands of people to march from the Sydney CBD across the bridge to North Sydney despite rainy conditions.
Police sought an order to prohibit the protest but Supreme Court Justice Belinda Rigg rejected the application on Saturday.
The bridge will be closed to motorists for most of Sunday and the metro is out of commission with scheduled repairs.
The original starting point of Lang Park in the city's CBD has been moved slightly north in negotiation with organisers to accommodate swelling numbers, and the march will end in Bradfield Park.
"I'm not sure the amount of numbers that are coming ... we couldn't really gauge the support (for the rally) so we have to deal with that as it progresses today," Acting Police Assistant Commissioner Adam Johnson told reporters hours before the march.
He warned police would take swift action against anyone who seeks to hijack the peaceful protest.
"That's our message all the time, whether it's a public assembly or not.
"I'm talking about anyone, I'm not talking about the actual protesters specifically, but anyone, that people are expected to obey the law."
Mr Johnson also raised safety concerns, citing the rainy conditions and the number of demonstrators.
"Crowd crush is a real thing ... but in this case, the risk is the numbers are unknown," he said.
"I've asked the police to be reasonable and tempered and measured as we always are but please listen to their instructions, and we will assist in a safe assembly."
Several Labor MPs will defy Premier Chris Minns and join the march alongside multiple Greens and independent colleagues.
The premier previously warned the city would "descend into chaos" if the protest went ahead.
Federal Opposition leader Sussan Ley questioned the shutting down of a "critical piece of infrastructure" in Sydney.
Formerly a pro-Palestinian MP in parliament before shifting her position, Ms Ley took aim at the protest organisers and suggested the rally be moved to another location.
"The language of the people running this protest doesn't sound peaceful to me," she told Sky News on Sunday.
"I respect the right of free speech and protest, but this is taking it to another level ... the protest could happen elsewhere."
Labor backbencher Ed Husic, who has been more outspoken on ending the war on Gaza, emphasised unity.
"What we are seeing is that Australians are deeply affected by the images they are seeing out of Gaza," he told ABC TV.
"They want to send a strong message through peaceful protests to governments, both here and abroad, that the killing has got to stop, the starvation has got to end."
Similar demonstrations are planned on Sunday in Melbourne and Adelaide.
More than 60,000 Palestinians have been killed in the war in Gaza, according to local health authorities, while the United Nations says dozens of people have died in recent weeks due to starvation.
Israel's military campaign began after militant group Hamas attacked Israel on October 7, 2023, killing 1200 people and taking more than 251 hostages.
