Hong Kong police seized cyanide stuffed in Pokémon toys and explosives at a children's learning centre as they arrested a chemistry teacher and another man over a rubbish bin blast in Wan Chai on the evening of this year’s banned July 1 march.

Officers said they found on Thursday hazardous substances at the education site in North Point, the workplace of one of the suspects, including potentially deadly cyanide hidden in “poké balls”. The suspects were identified as “Yuen”, 29, and “Lee”, 31.

“We strongly condemn such a behaviour in which the suspect made good use of the convenience at work to store dangerous items,” said Chief Inspector Chan Sin-woon of Wan Chai district’s crime squad.

“Such materials could endanger the lives of public members, especially children. It is totally irresponsible.”

Yuen, who said he is a clerk, and Lee, a chemistry tutor at the learning centre, were held for alleged possession of explosives and causing an explosion likely to endanger life or property. The latter offence carries a maximum sentence of life imprisonment.

Thousands of anti-government protesters returned to Hong Kong’s streets on July 1 – the 23rd anniversary of the city’s handover from British to Chinese rule – openly defying a ban on the annual rally to oppose Beijing’s newly imposed national security law.

At least 370 people were arrested that day, including 10 who were held by police officers exercising their new powers under the contentious legislation.

According to Chan, when police were dispersing crowds along Hennessy Road that night, a large explosion was heard outside C C Wu Building at 7.30pm and smoke was seen billowing from the rubbish bin.

A bomb disposal squad later found two burnt-out bottles containing white powder and the smell of bleach.

After studying CCTV footage, police arrested the two in their Ma On Shan and Kennedy Town residences on Thursday morning.

On raiding Lee’s home and workplace on Hong Kong Island, officers also found items including receptacles containing black-grey and white powder, brake oil and ammonia powder, as well as bleach products.

“We believe they were used to manufacture explosives. The bomb disposal squad found that the black-grey powder contained 40 grams of low-energy explosives,” Chan said, adding that further examination of the chemicals was needed.

She said the force would also investigate if the pair were in breach of the national security law.