That the lines of criminality had been redrawn became clear on Friday, when the authorities charged a 24-year-old man with terrorism and inciting separatism — the first person to be indicted under the new law. With a “Liberate Hong Kong” flag mounted on the back of his motorcycle, the man careened into a group of police officers on Wednesday, the anniversary of Hong Kong’s return to China from British rule.

Most years, that holiday draws large pro-democracy rallies. But this time, they were banned. Protests were scattered, and the police swept in and arrested hundreds. Ten people, including a 15-year-old girl, were accused of “inciting subversion,” a vaguely defined crime under the new law; some had merely waved flags, bearing slogans that had never been explicitly outlawed…

The police collected DNA samples and searched the homes of the 10 people arrested on suspicion of inciting subversion — measures that seemed excessive when applied to people accused only of possessing pamphlets, said Janet Pang, a lawyer who is helping some of them…

Shortly after noon on Thursday, a pro-democracy activist, Tam Tak-chi, emerged from the station, where he had spent the night after being detained. Mr. Tam met a young man inside who said he had been arrested after the police found a banner in his bag reading “Hong Kong Independence, the Only Way Out.” The man wept on his shoulder, Mr. Tam said.