The group of activists, known as the Hong Kong 12, were captured by Chinese authorities at sea in August and held incommunicado for months; 10 were sentenced in December by a Chinese court to prison terms of between seven months and three years. The other two, both minors at the time of their attempted escape, were returned to Hong Kong and remain in custody.

The sweep of the additional 11 on Thursday in a predawn raid underscored a deepening climate of fear since Beijing imposed a national security law in June that curtailed political freedoms in Hong Kong. The city’s leader, Carrie Lam, promised that the law would be used against a “very small minority” of people, but authorities have wielded their new powers broadly, arresting more than 100 people since then, including former lawmakers, moderate activists and others across the ranks of civil society.

Those detained Thursday include a lawyer, Daniel Wong Kwok-tung, who is also a district councilor, a local elected official. Wong, who helped many protesters arrested during anti-government demonstrations in 2019, also founded a restaurant in Taiwan that employs Hong Kongers living in exile there. A musician, Rono Fok, was also among those arrested.

National security officers conducted the arrests, according to social media posts and friends of those held, but police have not yet detailed if the alleged crimes fall under the security law.

People previously arrested under the security law and others held for politically related offenses in the city have had their passports confiscated, as authorities worry about the influence of the growing number of Hong Kongers in exile abroad.

Fears about the reach of the security law, which targets broadly worded offenses such as secession, subversion, terrorism and foreign collusion, prompted restaurants to remove pro-democracy posters and activists to disband their groups in recent months. In the wake of the law, some Hong Kong residents have left the city, fearing persecution. Some arrested for political crimes have skipped bail and are living without formal legal status in Western countries and Taiwan.

Others, including the 12 who tried to flee by boat, have tried other means of escape as the police had already confiscated their passports and travel documents.

In response to the crackdown, Western countries have tried to formalize pathways to residency and asylum for people fleeing Hong Kong, notably Britain, Hong Kong’s former colonial ruler.

The British government last year offered an avenue for some 3 million Hong Kong residents by allowing Hong Kongers who hold a British National (Overseas) passport, a holdover from the colonial era, to obtain U.K. residency and eventually citizenship.

The BNO measure will go into effect later this month. Leading pro-Beijing figures in Hong Kong have called for China to take reciprocal action to punish those leaving, including potentially forcing them to give up foreign citizenship.

Theodora Yu contributed to this report.