A planned protest across the Sydney Harbour Bridge has gone ahead after it was authorised by the Supreme Court just one day prior, in what organisers called a "historic" decision.

Tens of thousands turned out for the March for Humanity on Sunday despite torrential rain - with many carrying placards with messages to politicians to stop the war.

WikiLeaks founder Julian Assange was spotted among the protesters, with other notable attendees including federal MP Ed Husic and former NSW Premier Bob Carr.

The Sydney Harbour Bridge was last closed for a public assembly in 2023, when some 50,000 people marched over the iconic roadway for World Pride.

"Shame shame Israel, shame shame USA," the crowds chanted. 'What do we want? Ceasefire. When do we want it? Now."

Lots of families, many with small babies, came out to support the peaceful demonstration. Alongside them, stationed across the bridge were police officers from the riot squad.

"I know it's the other side of the world but it affects us here massively as well," says dad Alec Beville, comparing the children in Gaza with his three-year-old son Frankie who is on his shoulders. "We could be helping a lot more with aid," he adds.

"Our government hasn't put on any decent sanctions on Israel," says Zara Williams who's got her baby Avery in a sling. "We [Australia] just can't do nothing while there's a forced starvation of an entire population."

Two hours into the march, attendees received a text from NSW Police that read, "In consultation with the organisers, the march needs to stop due to public safety and await further instructions".

They have asked everyone on the bridge to stop walking north and turn back toward the city in a "controlled" way.

Police have not yet provided an estimate of the numbers attending the march.