“A geo-targeting alert has been issued to the Sydney CBD. If you receive a text message from +61 444 444 444, it is NOT a scam,” the tweet said.

“After consultation with protest organisers, we are asking everyone to stop walking north. As soon as the march has stopped, we will look at turning everyone around back towards to city but it needs to be done in a controlled way in stages to keep everyone safe.”

The force said in an updated post, that due to “public safety issues,” police will be “working with protesters to get everyone off the bridge safely, but in a staged matter”.

“Please remain calm and follow directions by police,” it said.

Protesters in Sydney had already begun crossing the Harbour Bridge.

Organisers have said on Instagram that “hundreds of thousands” of people had joined the demonstration. A NSW Police spokesperson told ABC News they were unable to estimate attendance numbers while the protest was underway.

Protesters had earlier predicted that tens of thousands would take part in the march across the Sydney Harbour Bridge. Source: AAP / AAP Image / Dean Lewins

Meanwhile, protesters in Melbourne have now marched down Swanston Street and are along Flinders Street approaching the King Street Bridge.

Protesters in Melbourne have begun their march. Source: SBS

Protest organisers Free Palestine Coalition Naarm have said the protest would finish at the bridge but have not publicised what action they intended to take at the intersection.

The ABC has reported that police in riot gear have now barricaded off the bridge.

— Zacharias Szumer