Anonymous threats sent by text message and email. GPS tracking devices placed under a car, and Chinese "funeral money" sent to an office. Ambushes by reporters working for state-controlled media. Accusations of disloyalty in the press.

These are some of the methods deployed in a campaign of intimidation being waged against lawyers in Hong Kong who take on human rights cases, have criticized a China-imposed national security law or raised alarms about threats to the rule of law. While some of Hong Kong's leading rights lawyers have been detained in the past two-and-a-half years, many others have become the target of a more insidious effort to cleanse the city of dissent — part of a wider crackdown by the ruling Communist Party on lawyers across China, say activists, legal scholars and diplomats.

Michael Vidler, one of the city's top human rights lawyers, is among them. Vidler left Hong Kong in April, a couple of months after a judge named his law firm six times in a ruling that convicted four pro-democracy protesters on charges of illegal assembly and possession of unauthorized weapons. Vidler interpreted the judgment as "a call to action" on the city's national security police "to investigate me," he said in an interview last month in Europe. He asked that his location not be disclosed.