The clampdown comes with Beijing's propaganda machine in overdrive to suppress news of the protests, which are expected to draw their biggest crowds yet as the former British colony begins a two-day public holiday Wednesday.
Chinese President Xi Jinping has stayed silent on the protests, but in remarks Tuesday night commemorating the 65th anniversary Wednesday of Communist China's founding, he insisted that "we must never separate ourselves from the people".
It was unclear whether the remarks were intended to refer to Hong Kong.
Since the dramatic escalation on Sunday of the Hong Kong protests, "a number of Chinese citizens have faced reprisals" for voicing their support, according to the overseas-based advocacy group China Human Rights Defenders (CHRD).
The group, which has compiled accounts from campaigners within China, said at least a dozen people had been detained and several others threatened.
Amnesty International put the figures even higher, saying at least 20 were detained and another 60 called in for questioning.
"The rounding up of activists in mainland China only underlines why so many people in Hong Kong fear the growing control Beijing has in their city's affairs," Amnesty's China researcher William Nee said in a statement.
The group called on Chinese authorities to "immediately release all those being detained for peacefully expressing their support for protesters in Hong Kong".
Those detained include activist Wang Long, who was taken away by police in the southern boom town of Shenzhen bordering Hong Kong on Monday for "creating a disturbance" after he posted messages about the protests online, CHRD said.
The 25-year-old Wang made headlines last month with his decision to sue a state-owned telecom operator for denying him access to US search engine Google.
Another activist, Shanghai-based Shen Yanqiu, posted online photos of herself with a shaved head in support of the Hong Kong protesters on Sunday, CHRD said. She was detained on Tuesday and is "being held in an unknown location", according to the group.
A group of "up to 20 citizens" were seized by police on Tuesday in the southern metropolis of Guangzhou, near Hong Kong, after gathering in a city park to voice support for the pro-democracy camp, according to CHRD.
At least two activists -- Huang Minpeng and Liu Hui -- were detained by police and "denied food" while in custody before they were released in the afternoon, the group said.
Other areas where activists have reportedly been detained or threatened include Beijing, the southwestern mega-city of Chongqing and central China's Jiangxi province.
At an event Tuesday night marking the anniversary, Xi made no mention of the Hong Kong demonstrations.
But he urged: "We must never waver in our faith and must never separate ourselves from the people."
"All problems that affect the party's creativity, cohesiveness and effectiveness must be addressed, all illnesses that harm the party's advanced nature and purity completely cured and all tumours grown on the healthy organism of the party removed," he added, according to the official Xinhua news agency.
State-run media was largely focused on China's National Day celebrations Wednesday, although the Communist Party mouthpiece People's Daily ran an unsigned opinion piece maintaining that the Hong Kong demonstrations constitute an "illegal assembly" that has "seriously disrupted social order".
The demonstrators "should bear legal responsibility" for their actions, it added.
The Communist Party has stepped up a campaign against dissent in the nearly two years since Xi rose to power.
Dozens of legal activists, journalists and academics have been detained or imprisoned.