BEIJING (AFP) - Authorities have detained a dozen activists across China and threatened several others who expressed support for Hong Kong's pro-democracy protests in recent days, a campaign group said on Wednesday.

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.

Since the Hong Kong protests' dramatic escalation on Sunday, "a number of Chinese citizens have faced reprisals" for voicing their support, according to the overseas-based advocacy group China Human Rights Defenders (CHRD), which has compiled accounts from campaigners within China.

They include activist Wang Long, who was detained by police in the southern boomtown of Shenzhen 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 for 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 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 south-western mega-city of Chongqing and central China's Jiangxi province.

Hong Kong leader Leung Chun Ying has faced mounting calls to resign and accusations of failing to engage with protesters after their "Umbrella Revolution" campaign for unfettered universal suffrage sparked the biggest civil unrest in decades.

Citizens in mainland China do not enjoy the same rights as those in the semi-autonomous region, however, and the ruling Communist Party has stepped up a campaign against dissent in the nearly two years since President Xi Jinping rose to power.

Dozens of legal activists, journalists and academics have been detained or imprisoned under Xi, provoking an outcry from foreign governments and rights groups.