Hong Kong authorities say they have seized “significant quantities of homemade explosives” and arrested more than a dozen people in raids across the city.

Separately, police were accused of overzealous responses at two protests on Sunday, arresting people and pepper-spraying journalists.

The pre-dawn raids on Sunday were in connection with an alleged campaign to get the government to close its borders with China to stop the spread of the coronavirus, Hong Kong police said.

The force said the raids across 22 locations found electrical equipment, 1.5kg of explosives and 2.6 tonnes of chemicals. Twelve men and five women aged from 21 to 53 were arrested.

A senior bomb disposal officer, Alick McWhirter, said officers retrieved three homemade bombs that were “close to completion”.

“Significant quantities of homemade explosives ready to be used were seized [along with] and an enormous quantity of precursor chemicals ready to be manufactured into yet more homemade explosives – over 2,500kg of chemicals”, McWhirter said.

He said Hong Kong had faced “an ongoing bombing campaign … that has been both violent and indiscriminate”. The campaign was designed “to intimidate in order to achieve political ends”, he said.

In late January, a small homemade bomb exploded at Caritas hospital, destroying a toilet cubicle and prompting the evacuation of some patients. In the following days unexploded devices were found at Shenzhen Bay control point and at a train station near the border with mainland China.

Social media messages reportedly claimed responsibility for the devices and made further threats while calling for the closure of borders with mainland China owing to the coronavirus.

Some in Hong Kong questioned the timing of the raids and accused police of being heavy-handed. There were also accusations, which the Guardian has not been able to verify, that some arrests were in connection with the pro-democracy protest movement that has swept the region since last June, and in particular the siege at Polytechnic University.

Trust in the Hong Kong government and police force has plummeted over the past six months. Last week, three high-profile pro-democracy figures were arrested including Jimmy Lai, the head of the popular newspaper Apple Daily, on charges of illegal assembly. Police were accused of taking advantage of a lull in protests owing to coronavirus fears to target key figures.

Also on Sunday, police were accused of a heavy-handed response to a protest in Tai Po, where people objected to the establishment of a clinic for suspected Covid-19 cases in a high-density residential area.

Several people were taken away by police, and attendees including journalists were pepper-sprayed. Prior to the protest police said the gathering was unlawful and warned against violence and “illegal acts of roadblocking”.

According to the South China Morning Post, protesters chanted pro-democracy slogans commonly heard over the past six months.

Man Nim-chi, an opposition district councillor for Tai Po, told the paper she was pepper-sprayed despite standing near journalists to act as an observer. “But one officer rushed in my direction and pepper-sprayed me in the face. It is sheer abuse of power,” she said.

Hui Nick Lam, a district councillor for Tai Po, told Hong Kong Free Press the police had gone “overboard”. He said: “The protest had barely started before the clearance operation.”

.@HKJA_Official has complained so many times about police pepper-spraying journalists even when there are no protesters in the vicinity. They’ve done it again today in Tai Po. #HKPoliceState pic.twitter.com/nwwehEU8cK — Kong Tsung-gan / 江松澗 (@KongTsungGan) March 8, 2020

On Sunday evening, large numbers of police and riot squad officers gathered in Tseung Kwan O in anticipation of a planned vigil for a 22-year-old student who died after falling from a building during last year’s protests. Reports and social media footage showed arrests, mass stop-and-searches, and kettling of crowds in an effort to disperse the memorial to Alex Chow Tsz-lok.

This policing is far more oppressive than 4mo ago. On Nov 8, 10k citizens freely made a shrine to Chow Tsz-lok at the spot where he fell inside Sheung Tak car park. Tonight just to get to car park one must run the gauntlet of hundreds of police & get stop&searched. #HKPoliceState pic.twitter.com/4AHpvJxDmj — Kong Tsung-gan / 江松澗 (@KongTsungGan) March 8, 2020

Police said they seized petrol bombs, glass bottles, lighters, a batch of bricks, umbrellas and helmets in a car park near the monthly vigil.