Western Australia Police say a man has been charged after a device containing screws and ball bearings was lobbed at a January 26 march in Perth.
The Invasion Day rally had attracted thousands of people to Forest Place in the CBD by midday on Monday, with speeches beginning soon after.
Around 12:30pm, police allege a 31-year-old Warwick man threw a glass object, approximately the size of "a medium coffee cup" containing an unknown liquid and shrapnel into the crowd.
"Members of the public ... observed a male throw an object down in front of the stage area," said WA Police Commissioner Col Blanch at a press conference on Monday.
"Those members of the public immediately spoke to police [and] they arrested the person who threw the device, and that male indicated that that device may contain explosives.
"Forensics are currently doing what they need to do to identify what that liquid is."
Commissioner Blanch thanked rally organisers for their help in "immediately" evacuating the area following the incident.
The man has been charged with making or possessing an explosive, and committing an act with intent to cause harm.
He will appear in Perth Magistrate's Court on Tuesday.
Clashes with anti-immigration marches
Police were out in force elsewhere around the nation, protecting against potential clashes between the March for Australia and Invasion Day groups.
In Melbourne, the two demonstrations snaked through the city only streets apart, with each ending where the other began.
"Please, do not engage," was the advice Gunnai and Gunditjmara activist Meriki Onus gave Invasion Day protesters ahead of potential run-ins.
Among those who addressed the Invasion Day rally was Millie Ingram, who read a statement on behalf of Uncle Mark Brown, a Burunong Elder.
Protesters then marched chanting "Always was, always will be" and "What do we want? Land rights. When do we want them? Now."
The largely peaceful Invasion Day march concluded near Flinders Street Station, where the March for Australia rally had begun earlier in the day.
At that protest, chants of "deportation" and "send them back" were heard as attendees waved Australian flags and carried signs supporting Pauline Hanson's One Nation party.
As the group moved towards the steps of parliament, loud boos erupted as protesters passed the Immigration Museum.
Riot and mounted police lined the streets, guiding demonstrators, including neo-Nazi members dressed in plain clothes.
