Hong Kong police said they have defused two homemade nail bombs discovered in the grounds of a school and are investigating whether the devices were linked to political unrest in the city.
Bomb disposal officers rushed to Wah Yan College in Wanchai district after a janitor noticed the devices, police said.
"The bombs were complete, fully functional and ready to be used," senior bomb disposal officer Alick McWhirter told reporters on Monday night.
Mr McWhirter said the two devices contained a total of 10 kilograms (22 pounds) of high explosives and that the remote control bombs were designed to be set off using a mobile phone.
Nails had been added to the bombs, which used a homemade explosive, to increase their destructiveness.
"Both of these devices have only one function, to kill and to maim people," Mr McWhirter said.
Police said they believed the college was not the target for the bombmaker and that the devices were likely being hidden there for later use.
Hong Kong has been engulfed by six months of sometimes violent protests – although there has been a significant reduction in clashes in the past three weeks. A massive march on Sunday passed peacefully.
Millions have marched in protests fuelled by years of growing fears that China is becoming more authoritarian over the area and is stamping out the city’s liberties.
Over the past six months protesters have used bricks, petrol bombs and even bows and arrows in their weekly confrontations with police who have fired nearly 16,000 rounds of tear gas and 10,000 rubber bullets.
Three protesters have been injured after being shot with live rounds.
Police said a small remote-controlled explosive device was detonated during clashes in the district of Mongkok in October. No-one was injured by that device which also used a mobile phone.
Senior superintendent Li Kwai-wah said police were investigating whether the two devices were linked.
The discovery of a bomb came a day after police said a semi-automatic pistol and more than 100 rounds of ammunition were discovered in a raid on protesters.
Five people in their early 20s appeared in court on Monday charged with intent to wound and were denied bail.
Two were also charged with unlawful possession of a firearm.