Officers said they were alerted a day earlier to a video shared on social media purportedly showing someone manufacturing explosives in an experiment outdoors.
The boy was apprehended in Tseung Kwan O the same day and has since been detained for investigation.
There were no injuries or damage.
Officers from the explosive ordnance disposal bureau seized chemical substances suspected to be explosive residue.
“While the experiment was conducted under a bridge, the site was right next to a public rest area and a jogging trail,” acting chief inspector Choi Ting-kwong said.
“He ignored his own safety and that of others by carrying out the experiment involving explosives in a public space.”
Choi reminded the public that manufacturing explosives is a serious offence.