A man has been arrested after allegedly throwing a device that police believe may have contained explosives into a crowd in Australia, police have said.

The incident occurred shortly after 12:30 local time (04:30 GMT) on Monday in a pedestrianised square in Perth, where the state premier said thousands had gathered for a rally supporting the rights of Indigenous people. It also coincided with the national holiday, Australia Day.

The 31-year-old man was detained at the scene and remains in custody. Police said the device did not detonate and no injuries have been reported.

It comes less than two months after an attack on Sydney’s Bondi Beach targeting a Jewish festival left 15 people dead.

Witnesses to Monday’s incident in Forrest Place said they saw the man throw an object towards the front of a stage while speeches at an Invasion Day rally were taking place, public broadcaster ABC News reported.

Western Australia Police Commissioner Col Blanch told reporters the device had not exploded but contained ball bearings and screws wrapped around an unknown liquid inside a glass container.

He said forensic investigators were in the process of identifying the liquid, describing the device as “very rudimentary” and about the size of a medium coffee cup.

Blanch said police “do not believe there is any ongoing threat to the community”.

Australia Day, on 26 January, marks the 1788 landing of Britain’s First Fleet in Sydney Cove and the start of colonisation. For many Indigenous Australians and their supporters, it is known as Invasion Day.

The contentious public holiday often sees large protests take place in major cities across Australia.

Western Australia Premier Roger Cook described the incident in Perth as “completely unacceptable”

“We can’t let hate win,” he said. “Now, more than ever, it is important that we treat each other’s views with respect. It’s our differences and diversity which has made Australia the country that we all love.”

Australia’s national terror threat level is currently considered “probable”, the third-highest rating on a five-point scale.

Several Australia Day events were cancelled or scaled back this year due to extreme heat, with parts of the country experiencing temperatures in excess of 40C.