Police have warned they will crack down on violent offenders as opposing left and right-wing groups plan to rally in Melbourne on Sunday.

The anti-Islam True Blue Crew plans to meet at 11.30am at Parliament House for "The Australian National Flag Solidarity Walk".

The United Patriots Front is supporting the rally, which will be met by a counter-protest by anti-racism groups.

Last month in Coburg, the opposing groups clashed at a multicultural anti-racism rally which erupted in violence.