Permission wasn't given for WikiLeaks protesters to march through Sydney because police weren't given advance notice, says Assistant Commissioner Mark Murdoch.
The protesters wanted to march along George Street, from Town Hall to the US Consulate in Martin Place, at 5.30pm (AEDT) on Tuesday.
But police rejected the application, saying they were not given the mandatory five days notice.
"The group gave one day's notice," Mr Murdoch told AAP.
"We did offer to facilitate a static protest; we also offered to facilitate an alternate route and offered to facilitate a march along George Street at a different time."
Mr Murdoch said the rally organisers were well aware of the requirements police had for protest action.
"There was a similar protest last Friday at the Town Hall," he said.
"Police explained the requirements that day to the organisers so they were well aware of what was required and these are
requirements for every group requesting permission."
The protesters have vowed to push ahead with the march.
"We're now having to defend our own freedom of speech to defend the freedom of speech of WikiLeaks," said Patrick Langosch, spokesman for the Support WikiLeaks Coalition.
"The right to protest, just like the freedom on information, is a basic human right...
"We want to march to the US Embassy to show we oppose their attempts to silence WikiLeaks... We can only conclude that the draconian actions of the NSW Police are political."
The Greens have also weighed in, calling on the government to give urgent permission for the rally - which is expected to attract more than 1000 people - to go ahead.
"Police have given no adequate reason to support this refusal. It appears that they would rather be minding Oprah than looking
after the legitimate rights of the public to freedom of expression," Greens MP David Shoebridge said in a statement.
"The Greens are calling on the minister for police to intervene and grant permission for the rally to go ahead."
