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.
"Forensics are currently doing what they need to do to identify what that liquid is," WA Police commissioner Col Blanch told reporters in Perth.
The device, about the size of a medium coffee cup, did not explode and nobody was hurt.
Officers searched the man's house. He is yet to be charged. Mr Blanch said there was no ongoing threat to the community.
While Invasion Day protesters used the day to call for better recognition of Australia's first peoples, large anti-immigration rallies were also staged under the banner "March for Australia".
One man who attended a rally in Sydney is facing charges of publicly inciting racial hatred, accused of spewing neo-Nazi talking points at a demonstration in Surry Hills.
"We will allege that the language he used, his presence, was clearly and unequivocally assigned with neo-Nazi ideology," NSW Police assistant commissioner Brett McFadden told reporters.
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.
Mr Brown's Welcome to Country at Melbourne's Shrine of Remembrance was heckled by neo-Nazis on Anzac Day in 2025.
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.
Police were quick to break up a clash after a man was seen attempting to burn the Australian flag, prompting another man to step in and hurl abuse.
Victoria Police said it was investigating three assaults believed connected to the rallies, including a couple having their car attacked with a broken boom gate while being racially abused by four men, with one performing Nazi salutes during the ordeal.
Similar protests were held across all capital cities, including in Brisbane, where Senator Hanson took to the stage.
"I've never felt so honoured and proud as I am to be here today," she said to an enthusiastic crowd.
"Thank you for flying the Australian flag and showing pride in our country."
In Sydney, March for Australia protesters shouted chants of "Aussie, Aussie, Aussie - oi, oi, oi" amid a heavy police presence.
Protesters were draped in Australian flags while some held signs reading "Free Joel Davis", referring to a neo-Nazi on remand for allegedly harassing a federal MP.
An Invasion Day protest earlier kicked off in Hyde Park, opening with a minute's silence and a tribute to the victims of the recent Lake Cargelligo shooting.
Palestinian flags and keffiyehs were visible among Indigenous flags and placards.
NSW Premier Chris Minns said he was hopeful the protests would proceed peacefully, adding there would be "no tolerance for violence or hate speech on Sydney's streets."
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