Australia Day protests around the nation have been marred by a bomb threat, assaults and hateful language, with two men arrested over separate incidents in Perth and Sydney.

In Perth's CBD, Invasion Day demonstrators were told to leave the area after a man threw what's believed to be a rudimentary explosive device into the crowd.

Police said they arrested a 31-year-old man and recovered a device made of ball bearings and screws, wrapped around an "unidentified liquid" in a glass container.