Police have detained a 20-year-old student after finding a kilogram of potassium chlorate, an explosive material, on his person.
It was discovered in eight pieces on the man at the Admiralty Centre. He was heading to a rally at the legislature against a controversial copyright bill, police said.
The ingredient generates smoke when burned and is used in fireworks and gunpowder.
Lighters, gloves and surgical masks were also found on the man, who was not named by police.
On Wednesday evening, hundreds gathered at the rear of the LegCo building to demonstrate against the bill which campaigners fear may criminalise those who use copyrighted work for personal or non-profit use.
The debate on the Copyright (Amendment) Bill 2014 was delayed once again by pan-democrats until 9am Thursday morning.