China's military forces will move in to "quell" the pro-democracy protests in Hong Kong if the situation deteriorates, an adviser to Hong Kong's leader Carrie Lam has said.

Key points: Mr Zeman says Chinese forces may take the place of Hong Kong police who are not trained to handle the demonstrators

Mr Zeman says Chinese forces may take the place of Hong Kong police who are not trained to handle the demonstrators Chinese military forces have been training across the border

Chinese military forces have been training across the border Hundreds of thousands of Hong Kong protesters rallied peacefully in torrential rain on Sunday

Hong Kong has been the scene of increasingly violent demonstrations, which have disrupted everyday life in the city and led to multiple closures of its international airport.

The rallies began in June as protests against a now-suspended bill that would allow people suspected of a crime to be extradited to mainland China for trial in Communist Party-controlled courts but now encompass broader calls for democracy in Hong Kong and for Ms Lam to step down as chief executive.

"If it got to a point that really endangered Hong Kong and its notion of 'One Country, Two Systems', I am sure the People's Liberation Army would come in and hopefully quell some of whatever was going on," Allen Zeman, an economic adviser to Carrie Lam told the ABC.

Mr Zeman said Hong Kong police had not been trained to handle the mass demonstrations and many of them were "learning on the job".

"Hong Kong doesn't have an army, we have the police. This is something that was thrown on them," he said.

His comments echoed the words of Zhang Xiaoming, the director of the Hong Kong and Macao Affairs Office of the State Council, who said in an August 7 press conference China would not sit idly by if the unrest worsened.

Mr Zhang said China had sufficient capabilities to quickly subdue demonstrators.

Members of China's paramilitary forces have been seen training across the border in Shenzhen, fuelling speculation they could be sent in to suppress the protests.

"Hong Kong does belong to China, whether we like it or not," Mr Zeman said.

"We have to work within the Chinese framework. They really don't want to clamp down on freedom here."

He added while "many people around the world think that Hong Kong is in total chaos", it was still "business as usual" in much of the city.

Hong Kong leader offers protesters an 'olive branch'

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 44 seconds 2 m 44 s Hong Kong's citizens set to reclaim their peaceful 'People's Movement' for democracy and independence

Over the weekend, up to 1.7 million anti-government protesters of all ages braved torrential rain to peacefully rally in Hong Kong's Victoria Park and its surrounding roads.

It was one of the biggest protests held since the initial June 16 march.

Ms Lam said on Tuesday she hoped the peaceful demonstrations were the start of efforts to restore calm, and talks with non-violent protesters would provide "a way out" for the Chinese-ruled city.

"I sincerely hope that this was the beginning of society returning to peace and staying away from violence," Ms Lam said.

"We will immediately start the work to establish a platform for dialogue. This dialogue, I hope, will be based on a mutual understanding and respect and find a way out for today's Hong Kong."

Mr Zeman said Ms Lam had offered protesters an "olive branch", but many of their demands were unreasonable.

British consulate employee missing in China

Ms Lam held her press conference on the same day reports surfaced an employee of the British consulate in Hong Kong had been "administratively detained" by Chinese authorities.

Simon Cheng Man-kit, a trade and investment officer at the Scottish Development International section of the consulate, attended a business event in Shenzhen on August 8 but did not return to neighbouring Hong Kong despite plans to do so the same day.

Mr Cheng's family said the Immigration Department told them Mr Cheng had been detained, but they were unable to find out why, where or for how long he would be detained.

"We feel very helpless and are worried sick about Simon," the family posted on Mr Cheng's Facebook page.

"We hope that Simon can return to Hong Kong as soon as possible."

Loading

A statement from Britain's Foreign and Commonwealth Office said it was seeking information from Hong Kong and Guangdong province about the man's detention.

"We are extremely concerned by reports that a member of our team has been detained returning to Hong Kong from Shenzhen," the statement said.

"We are providing support to his family and seeking further information from authorities in Guangdong province and Hong Kong."

Sino-British relations have grown tense in recent months amid pro-democracy protests in Hong Kong.

ABC/wires