The Hong Kong police have defused two home-made bombs found at a secondary school.

Police said the bombs contained ten kilograms of explosives and were complete and fully functional. They were able to cause casualties within a range of 50 to 100 metres, the force said.

Police Organised Crime and Triad Bureau Senior Superintendent Steve Li said they received a call from a janitor of the Wah Yan College Hong Kong in Wan Chai at around 5:30pm on Monday reporting the discovery of the devices in the grounds of Gordon Wu Hall.

Police found ten kilograms of hexamethylene triperoxide diamine and ammonium nitrate powders in the bombs in sieves – both highly explosive. Metal nails, mobiles phones and electrical boards were also discovered.

“We are examining who put the bombs there, [and] why inside the school,” he said. “We also have to see whether the case was related to recent violent incidents, especially the conspiracy to wound and gun case.”

On Sunday, police arrested seven males and four females on suspicion of possession of a firearm without a licence, possession of dangerous goods, possession of prohibited weapons and unlawful assembly. Five of them have been charged.

Police Senior Bomb Disposal Officer Alick McWhirter said the bombs were “complete, fully functional and ready to be used.”

“Both of these devices have only one function – to kill and maim people,” he said. “Given the quantities of the explosives and the fragmentation, had these devices been placed, and had they functioned, they would have killed and injured large numbers of people.”

In October, a suspected improvised explosive device was found in Mong Kok near police vans during a protest. Pro-democracy protests and unrest have been ongoing since June.

Wah Yan College Hong Kong said in a statement that the school did not know about the situation until the police arrived.

It said the location that the devices were found was around a private property that the school owned, but it was outside the school gate so was accessible to the public.

“There was no evidence to show any connection to our students or teachers or staff members. We hope the police will find out the truth as soon as possible,” it said.

Classes proceeded as normal on Tuesday.

Correction: A previous version of this article suggested the area in which the bomb was discovered was inaccessible to the public. It is, in fact, open to the public.