State Security Investigation Group officers have charged a man following an investigation into the attempted bomb attack in Boorloo/Perth on Monday.

WA Police said the man was allegedly in attendance at the Survival Day / Invasion Day rally at Forrest Place, where a crowd of approximately 2,500 people were present.

Police allege the man removed an item from his bag and threw it into the crowd from a first-floor walkway before fleeing the scene.

A member of the public alerted police, who took immediate action to protect and disperse the crowd and to isolate the suspicious item.

The man was identified and taken into custody by police on the first-floor walkway.

Specialist officers from the WA Police Bomb Response Unit and forensic officers attended the scene and examined the item. It was confirmed to be a homemade improvised explosive device containing a mixture of volatile and potentially explosive chemicals, with nails and metal ball bearings affixed to the exterior.

A subsequent search of the man's home was conducted, where it is further alleged a combination of chemicals and materials consistent with the manufacture of homemade explosives was located.

The 31-year-old Warwick man has been charged with one count of 'Unlawful act or omission with intent to harm (endangering life, health or safety of any person)', and one count of 'Making or possession of explosives under suspicious circumstances'.

His bail was refused, and he is due to appear before the Perth Magistrates Court, Tuesday 27 January.

WA Premier Roger Cook described the attack as "completely unacceptable".

"Now more than ever it is important that we treat each other's views with respect. It is our differences and our diversity which make Australia the country we all love," he said.

"We can't let hate win. It is completely unacceptable in Australia. It is completely unacceptable in Western Australia."