Hong Kong police use pepper spray at anti-China protest

In chaotic scenes reminiscent of mass pro-democracy protests in 2014, demonstrators charged metal fences set up by police outside China's liaison office in the semi-autonomous city .

Protesters used umbrellas to shield themselves from the spray as they tried to approach the building, while some threw water and water bottles at police.

"We are quite shocked, we just wanted to express our demands at the liaison office," Cas Wong, a 19-year-old student, told AFP.

There are deep concerns those liberties, enshrined in the city's constitution known as the Basic Law, are under threat.

In Sunday's demonstration protesters marched to the city's court of final appeal and the Chinese liaison office, carrying banners reading: "Chinese law interpretation tramples on Hong Kong people."

"Hong Kong has its own legal system, it should not be controlled by Chinese authorities," protester Alex Wong told AFP.

"We must tell the government we are not happy," the 35-year-old office clerk said.

Monday's expected announcement will mark the fifth time since the handover that China has interpreted Hong Kong's constitution.

The lawmakers at the heart of the controversy have previously said the interpretation would deal a "lethal blow" to the city and branded the chairman of the Chinese legislature's standing committee and the city's leader "traitors to Hong Kong".

Yau and Leung won seats in September's citywide polls, in which several new lawmakers advocating self-determination or independence swept to victory.

At least one person was seen taken away by multiple officers before scuffles broke out.

Police warned protesters they were involved in an "unlawful assembly" and told them to disperse, as they donned riot gear.

The protest began with a peaceful city centre march in the afternoon. Organisers said 13,000 people took part in that, while 4,000 people rallied outside the liaison office.

Police estimates were not immediately available.

Hong Kong's pro-Beijing government is trying to bar newly appointed pro-independence legislators Baggio Leung and Yau Wai-ching from taking their seats in the Legislative Council (Legco) after they deliberately misread their oaths of office last month.

As fears grow that Beijing -- to whom any independence talk is treasonous -- is tightening its grip on the city, China has announced that its parliament will interpret Hong Kong's constitution, which states that council members must swear allegiance to the "Hong Kong Special Administrative Region of the People's Republic of China".

The decision of the National People's Congress Standing Committee is expected to be announced on Monday.

China is reviewing the issue even though a Hong Kong court is also considering the matter, a move which local lawyers say erodes the independence of the city's judiciary.

Hong Kong was handed over by Britain to China in 1997 under a "one country, two systems" deal which guaranteed its rights and freedoms for 50 years.