HONG KONG (BLOOMBERG) - Hong Kong Chief Executive Carrie Lam received a letter that contained a blade, the government said, a sign of the tensions that linger in the city after Beijing imposed a national security law.

The intimidating letter was discovered during a routine check of Mrs Lam's mail, according to a statement late on Monday (Dec 13) from a government spokesman, which did not provide further details.

The South China Morning Post reported that it was a razor blade.

"(The city) will not tolerate such illegal acts as violence and intimidation," the spokesman said.

"(And officials) will take the case seriously and spare no effort in bringing the culprit to justice to safeguard the safety of public officers and public peace."

Fears of violence in Hong Kong have lingered since July 1, when a man stabbed a police officer on a busy shopping street on the anniversary of the former British colony's return to Chinese control.

The assailant died after stabbing himself in the chest, and the officer survived.

Mrs Lam has been spearheading an unprecedented national security crackdown on the Asian financial centre. She has said the security law imposed in the summer of 2020 eliminated "the pervasive violence and social unrest that created anxiety" in Hong Kong, referring to large protests a year earlier.

In July this year, police arrested a group of people they said were planning bomb attacks.

The suspects wanted to attack a major road tunnel, railways and courts, a police official said. Several of them have been charged under the security law with "conspiracy to commit terrorist activities".