Members of China's paramilitary People's Armed Police marched and practised crowd control tactics at a sports complex in Shenzhen across from Hong Kong on Friday, in what some interpreted as a threat against pro-democracy protesters in the former British colony.

The sound of marching boots and synchronized shouts echoed from the grounds. Officers in green camouflage stood guard at closed entrances. A stadium security guard said "it wasn't clear" when the paramilitary police would leave the grounds.

Chinese state media have only said the Shenzhen exercises were planned earlier and not directly related to the unrest in Hong Kong, though they came shortly after the central government in Beijing said the protests were beginning to show the "sprouts of terrorism."

From a distance, police could be seen conducting drills in military fatigues, using shields, poles and other riot-control gear. In one exercise, two groups marched in formation with those in front raising shields as if to protect themselves from projectiles. Others behind held red flags and banners. The words "the law" and "prosecuted" could be seen on one.

Chinese soldiers walk in formation on the grounds of the Shenzhen Bay Sports Center on Thursday in Shenzhen. (Thomas Peter/Reuters)

Outside, dozens of armoured carriers and trucks sat in the parking lot of the Shenzhen Bay Stadium, close to a bridge linking mainland China to Hong Kong.

Asked if Hong Kong police could maintain order or if mainland Chinese intervention is becoming inevitable, Hong Kong police commander Yeung Man-pun said that while they face tremendous pressure, "I can tell you we're confident the police have the capability to maintain law and order."

Weeks of protests and clashes

Weeks of protests in Hong Kong have been marked by increasing violence and a shutdown of the Hong Kong airport earlier this week. The demonstrators are demanding expanded political rights and the scrapping of legislation that could have seen criminal suspects sent to mainland China.

A weekend of protests began Friday night with a university student-led "power to the people" rally in Chater Garden, a public square in the financial district.

A breakdown of the demographics of protesters participating in the latest demonstrations in Hong Kong. (CBC News)

A pro-democracy march is planned for Saturday along with a separate pro-government Save Hong Kong rally, ahead of a major pro-democracy rally called for Sunday. Police have denied permission for the march on Sunday, but protesters have ignored such denials in the past.

China has pressured foreign and Hong Kong companies to support the ruling Communist Party's position against the protesters.

Head of main airline resigns

The CEO of Cathay Pacific Airways, one of Hong Kong's most prominent companies, resigned Friday following pressure by Beijing on the carrier over participation by some of its employees in the anti-government protests.

Cathay Pacific said Rupert Hogg resigned "to take responsibility" following "recent events."

The company chairman, John Slosar, said in a statement the airline needed new management because events had "called into question" its commitment to safety and security.

On Monday, Hogg threatened employees with "disciplinary consequences" if they took part in "illegal protests.'

Police and protesters clashed at Hong Kong's international airport Tuesday evening after flights were disrupted for a second day. 0:59

Last week, China's aviation regulator said Cathay Pacific employees who "support or take part in illegal protests, violent actions, or overly radical behaviour" are banned from staffing flights to mainland China.

On Friday morning, Frenchman Alain Robert, who has been dubbed "spiderman" for his unauthorized climbs of skyscrapers, hung a banner appealing for peace as he scaled the 62-storey Cheung Kong Center, a landmark Hong Kong building that is the base for property tycoon Li Ka-shing's business empire.

Graph shows which demands and issues matter most to protesters in the span of weeks of demonstrations in Hong Kong. (CBC News)

The banner showed the Chinese and Hong Kong flags over a handshake and a small yellow sun with a smiley face.

"The banner is to give joy and smile to the people of Hong Kong," he told the AP as he sat in a taxi about to leave for his climb. He said he didn't want to get "mixed up in the political situation."

Robert, 57, was taken to a police station afterward. It wasn't immediately clear if he would be charged. He was banned in 2017 from returning to Hong Kong for one year after climbing another building.