The Sydney 'Stop All Black Deaths in Custody' rally has been declared an authorised public assembly after a late decision by the Court of Appeal.
The decision came 12 minutes before the rally's scheduled start at 3pm outside Town Hall today.
It means protesters - already gathered in many hundreds - cannot be arrested for blocking roads along the planned route from Town Hall to Belmore Park.
The appeal was heard from 2pm after organiser Raul Bassi asked the Court of Appeal to overturn the decision of Supreme Court Justice Desmond Fagan made on Friday night.
Justice Fagan found the document Mr Bassi filed with police last week was "entirely different" to an amended notice filed on Thursday increasing numbers from 50 to 5000.
He refused to approve the public assembly, citing the current coronavirus restrictions on mass gatherings.
"Our respectful submission is that the court will now correct that error and the declaration will be made that it was an authorised public assembly," barrister Stephen Lawrence said.
Michael Spartalis, for the Commissioner of Police, said the court's decision last night wasn't appealable under the summary offences act.
He added there was no evidence police had agreed to the Town Hall assembly.
Chief Justice Tom Bathurst said the reasons for the decision would be published early next week.
Mr Bassi did not seek costs.
Greens MP David Shoebridge earlier said the organisers had "received strong" legal advice that Justice Fagan's ruling had "significant flaws" that amount to jurisdictional error.
Mr Bassi said he had planned to have marshals monitor social distancing and hand out face masks and hand sanitiser.
Town Hall railway station has been closed and there is a large police presence in the city monitoring the rally.
One man holding an "All Lives Matter" banner was moved from the area by police.
NSW Chief Health Officer Kerry Chant on Friday said a protest of 10,000 could "increase the risk of community-acquired infections and the seeding of clusters".