But a small cohort of mainlanders have joined the demonstrations, taking extraordinary risks to support a society that offers freedoms unavailable back home.

“My understanding is that ‘one country, two systems’ is a creative set of ideals,” Ms. Chen said. “Now those ideals are threatened.”

In more than a dozen interviews, these outliers said they value Hong Kong’s autonomy from Chinese control, especially when it comes to the city’s legal system and freedom of expression. They have joined marches, signed open online letters supporting Hong Kong and defended the movement in social-media battles against state-backed critics and misinformation.

Those actions put them at risk of detention by Chinese authorities, who are checking travelers’ smartphones at the border crossing to Shenzhen for evidence of participation in the demonstrations. Mainlanders living in Hong Kong have also experienced ostracism from friends back home who see the protesters as a violent fringe, especially after they beat up a pair of mainland citizens in a melee at the airport. The incident set off a wave of anger in China and a bout of soul-searching among the protesters.