Victoria Police urges members of the public to consider their movements around Melbourne's CBD as four separate groups stage demonstrations on Saturday.
The force said intelligence shows those who hold "far-right and extreme ideologies, as well as far-left and opposing views" will seek conflict and confront each other.
Troublemakers have been warned they will be arrested, while others planning to join the rallies have been advised to consider their plans because violence could erupt.
Some 20 people were arrested when anti-immigration and counter-protesters clashed in Melbourne on August 31, with police using pepper spray to separate the groups.
Neo-Nazis attended the anti-immigration protest and a prominent figure addressed a crowd of thousands of people on the steps of state parliament.
Afterwards, a group of men dressed in black stormed Camp Sovereignty, an area in King's Domain where the remains of Indigenous people from 38 clans are located.
Police said individuals targeted officers with hostile behaviour during the earlier city clash, including throwing liquids and projectiles and yelling obscene messages.
Rally leaders who worked with officers in the lead-up to the events disregarded safety advice and changed their plans on the day, they added.
People are sick of protesters using rallies as a "veiled excuse to punch on", Victoria Police Superintendent Troy Papworth said.
"Throwing rotten fruit, taunting police, and violently attacking people based on their race is not  protesting - it's gutless," he said.
"If you are coming into the CBD on Saturday with the intention of fighting, throwing objects or abusing others, you can expect to spend some time in a police cell."
Police have extra powers to search people for weapons in the CBD on Saturday and can direct people to remove face coverings.
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