Security staff evacuated eight people from a Hong Kong court building on Thursday after a threatening letter containing a highly corrosive substance was sent to a judge who had recently jailed five university students for their roles in a major clash during the 2019 anti-government protests.

Police said they received a call from security staff at West Kowloon Court in Cheung Sha Wan at about 4.20pm over the suspicious letter.

The bomb squad was sent to examine the powered substance and found there was “no danger” of an explosion. The powder, which was later confirmed to be about two grams of caustic soda, was wrapped in aluminium foil. A foul language curse written on a piece of paper was sent with it.

Officers removed the substance for further checks.

The letter was sent to Kathie Cheung, a source said.

A source said the letter was addressed to Kathie Cheung Kit-yee, a deputy District Court judge.

Cheung handed the item to an assistant who took it to the security department.

The suspicious letter triggered an evacuation at the Tung Chau Street building, with police saying eight people were taken to safety. No one was injured.

The case has been classified as criminal intimidation and Sham Shui Po district crime squad is investigating.

A Department of Justice spokesman said the government would not tolerate any act of intimidation against judges.

“Such illegal acts not only disregard the law but also undermine the rule of law. Police will follow up on the matter seriously,” the department said.

It said that under Article 85 of the Basic Law, the city’s mini-constitution, courts should exercise judicial power independently, free from any interference, while judges should handle cases in accordance with the law and evidence.

Intense clashes occurred at the Chinese University campus in November 2019.

Anyone convicted of threatening to injure another person faced five years’ imprisonment, it warned.

Cheung last month sent five current or former Chinese University students to jail for up to 59 months each for rioting and flouting a ban on wearing masks at public gatherings during a violent protest on the campus in November 2019.

It was the second time a judge had handed down prison sentences over the intense clashes between hardcore protesters and riot police that gripped the Sha Tin campus at the height of the anti-government unrest.

Caustic soda, or sodium hydroxide, is a highly versatile substance used in a variety of manufacturing processes. It is highly corrosive in all forms and can cause serious harm.

Law Society president Chan Chak-ming said intimidating judges was not only a serious criminal offence, but also posed a threat to the rule of law and judicial independence and had to be stopped immediately. The society urged all parties to respect the rule of law and the constitutional role of judges.