Marchers at Perth's Invasion Day rally have been told to evacuate by police after reports of a threat, with one person in custody.
WA Police confirmed officers are responding to reports of a threat in Forrest Place in the CBD.
"Significant" police resources are on the ground in the area and an exclusion zone has been established.
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One person has been arrested and is speaking with police.
Members of the public have been urged to stay away from the area.
It has been reported there are concerns someone has thrown something dangerous into the crowd.
Two separate rallies are currently under way in the Perth CBD to mark Australia Day.
An Invasion Day rally began at 12pm local time (3pm AEDT) in Forrest Place and protesters gathered for a March for Australia demonstration in Wellington Square at the same time.
More to come.