A homemade "fragment bomb" thrown into a crowd at a Perth Invasion Day rally in Australia yesterday was hidden in a child's Disney-themed sock, according to a woman who picked it up.

Axar.az reports, citing ABC, police alleged the device was packed with chemicals and designed to explode on impact, but it did not detonate.

It was thrown from a shopping centre balcony into the crowd, causing police to evacuate up to 2,500 people who they said had gathered for the January 26 event in central Perth.

Police Commissioner Col Blanch described the device as a "fragment bomb".

"We're still waiting for full chemical analysis, but I can confirm the preliminary tests showed that there were three compounds involved," he told ABC Radio Perth.

"We are able to say that it is an explosive device, we will call this an improvised explosive device.

A man was due to face court today in relation to the incident.

He is charged with intent to do harm in such a way as to endanger life, health or safety, and with making or possessing explosives under suspicious circumstances.