Police interrupted the crowd and moved people back from the stage following a "threat". Photo: ABC News / Cason Ho
Despite the mild threat of summer rain, hundreds of people descended on Forrest Chase, a common meeting place in the heart of Perth's CBD, for an Invasion Day rally.
But about half an hour into speeches from Aboriginal leaders, the peaceful gathering turned to chaos.
Police officers and organisers quickly started telling attendees to move away from the stage.
"I don't want to make a panic. Just move that way slowly," one organiser yelled into the microphone.
"Because they reckon someone planted a bomb. Walk that way now. Now."
You need to move. Your safety is paramount," an officer told the gathering.
Some in the crowd were confused about the instructions, with some insisting on continuing the rally in place.
No-one really told us what was going on, and it was kind of stressful," Lilli O'Flyn, who was in the crowd, recalled.
"But just because no-one was communicating with us, we didn't really know what to do."
About 300 people gathered in Perth's CBD on Monday. Photo: ABC News
Ms O'Flyn said she quickly decided to move away from the crowd with a friend "because when you don't know, you go to worst-case scenario".
'Potential mass-casualty event'
What few in the crowd knew was that the evacuation had been triggered by attendees on a walkway above Forrest Chase seeing a man allegedly throw an item towards the stage.
"Police immediately responded. They arrested the person who threw the device, and that male indicated that that device may contain explosives," Commissioner Col Blanch said in the hours after the event.
Officers surged to the area, responding to what Commissioner Blanch described as a "potential mass-casualty event", while those already on scene quickly moved the crowd away.
Among them was Ben Quick, who said officers formed a line shouting "move" and "halt" to control the speed of the crowd, while threatening arrest if people did not comply.
Ben Quick said he heard police shout "move" during the rally. Photo: ABC News / Keane Bourke
Commissioner Blanch said nobody else had been arrested at the rally, and thanked attendees and organisers for their help in keeping the evacuation orderly.
With the crowd cleared as the rally made its way through the city, officers were able to inspect the item.
Ball bearings, screws and a mystery liquid
Commissioner Blanch described it as "fairly rudimentary" and about the size of a medium coffee cup.
"The device that was located just in front of the stage area contained ball bearings, contained screws, and those items were wrapped around an unknown, at this stage, liquid in a glass container," he said.
"We're in the process of identifying the liquid."
Officers raced to the man's Warwick house, which was searched while he was being interviewed.
A large section of the CBD remained cordoned off for about two hours, with confused crowds gathered at all sides.
'We must remember what this day marks'
Around an hour after the cordons were lifted, Premier Roger Cook and Commissioner Blanch addressed the media.
With Australia Day celebrations, including fireworks, planned for the hours ahead, Commissioner Blanch reassured the community there was no ongoing threat, with all indications pointing to the man acting alone.
"We are at that person's house conducting a thorough and full investigation on any devices that may be present at that house and other items there," he said.
Police described the threat as "a potential mass casualty event". Photo: ABC News / Keane Bourke
Work was expected to continue through the night, including identifying if the liquid around the screws and ball bearings was explosive and what the man's motives may have been.
"Whatever the motivation for this, we must remember what this day marks," Premier Roger Cook said.
"This Australia Day should be about unity, not division.
"That a peaceful protest was targeted in this fashion runs against the very heart of what it means to be Australian."
Festive atmosphere in Langley Park for fireworks
Meanwhile, on the banks of the Swan River, the mood was much more carefree for those who turned out to watch the fireworks.
The festivities at Langley Park began at 3pm with carnival rides, a sideshow alley and a multicultural zone that celebrated the west's diversity.
The crowd was entertained by the daredevils of the Freestyle Motocross (FMX) and BMX stunt shows, as daring riders flew through the air in breathtaking fashion.
Perth produced mild summer conditions - in contrast to much of the nation that was baking in near-record heat - with the temperature dropping towards 20 degrees Celsius as the sun set and the healthy crowd awaited the fireworks.
The light show arrived on the dot at 8.30pm.
The West Australian capital may have been the last major city across the nation to produce Australia Day fireworks, but it was worth the wait.
-ABC
