Rival protest groups plot to clash with each other in the heart of a major city, say police who brand the troublemakers among them as "gutless".

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

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