Home-made bombs using pressurised gas canisters were seized by Hong Kong police near a protest site in Mong Kok on Thursday night, which the force believed were intended to attack officers.

Police also arrested a man on Friday morning for selling weapons online as it was revealed a protester had used a stun gun during an attack.

The force described the situation as “worrying” and a “bad omen” if such dangerous weapons were falling into the hands of those with “ill intentions”.

Anti-government protesters gathered in Mong Kok and Central on Halloween to protest against alleged brutality by police and in open defiance of a mask ban. Police fired 125 rounds of tear gas and 28 rubber bullets over the course of the evening to disperse the crowds.

During a media briefing on Friday, the force said officers in Mong Kok Police Station spotted a suspicious truck unloading bags of items on the flyover of Prince Edward Road West at 11.30pm.

Officers later discovered 10 aluminium foil thermal bags containing cotton and dozens of pressurised gas canisters on the road.

A police insider said the cotton was soaked in fuel which would burn slowly, heating up the canisters to an explosion. He believed the bombs were intended to hurt officers as there was a police check point under the flyover.

“This kind of bomb – made with gas canisters more commonly used for hotpot – is highly dangerous and destructive when ignited,” said Chief Superintendent John Tse Chun-chung from the force’s public relations branch.

Tse added that it was the first time police had seized explosives using flammable compressed gas.

“We express the strongest condemnation of this escalating violence,” he said.

About two weeks ago, a home-made remote-controlled bomb went off near a police car in Mong Kok. The device comprised of a mobile phone, a circuit board, a battery and some high-powered explosives.

Tse also said officers mounted a covert operation on Friday morning and arrested a man selling extendable batons and stun guns online. A total of 20 batons and 10 stun guns were seized.

The night before, a mob had surrounded a man during the illegal protest in Mong Kok and beat him and stamped on him until he was unconscious.

“One rioter used a suspected stun gun to attack the victim, who was left with bloody and life-threatening injuries. This dangerous violence is monstrous,” Tse said.

Separately, Tse accused protesters of using Halloween as an excuse to hijack public spaces where people were enjoying the occasion. Protesters blocked roads, set fires and assaulted innocent bystanders, he said.

Police enforced crowd control in Lan Kwai Fong last night after 7pm due to overcrowding. Senior Superintendent Ng Lok-chun explained that police had exercised such measures in the district for Halloween for many years. There was a stampede [on New Year] in 1993 where 20 people died.

On Wednesday night, police officers entered Yat Sang House, a private apartment building in Tuen Mun and subdued individuals inside, sparking uproar.

Senior Superintendent Kelvin Kong Wing-cheung said the police had the right to enter any building if they had reason to believe a person to be arrested was inside.

Kong explained this was also why police had tried to pursue two people who had disturbed public order when they escaped into a restaurant on October 30. When the owner and a member of staff denied officers entry, they were also arrested.

Hong Kong is about to enter its 22nd straight weekend of protests, first ignited by a now-withdrawn extradition law.

Since last Monday, police have arrested 249 people, the youngest of whom was 13. They were suspected of being involved in unlawful assembly, possession of offensive weapons and breaching the anti-mask law.

In total, since June, police have arrested nearly 3,000 individuals and fired about 6,000 rounds of tear gas.