HONG KONG • Hundreds of masked protesters used bins and ticket machines as barricades against riot police late yesterday at a flashpoint Hong Kong metro station, where they earlier marked one month since a mob attack by suspected triad gang members.

That assault on July 21 had pushed the city deeper into political violence, but the sit-in yesterday was largely peaceful until police vans arrived at the Yuen Long station close to the Chinese border and the atmosphere soured.

Some protesters put on hard hats and gas masks, while others sprayed fire extinguishers and poured detergent, beer and oil on the concourse to impede entry by the police, who had massed outside.

The demonstration was convened to mark a month since a gang wearing white T-shirts and armed with poles and sticks set upon anti-government protesters - unarmed and dressed in black - at the metro station.

The attack left nearly 50 people - including passers-by - in hospital, some with horrific wounds.

Hong Kong's police were heavily criticised for being slow to respond, fuelling rumours of collusion. Police denied any links to the attack, but trust in the force - which was already facing an unprecedented challenge on the streets - has since sunk to a nadir.

The police on Tuesday said they had arrested 28 people over the July 21 incident.

Yuen Long is in Hong Kong's New Territories, a rural area where many of the surrounding villages are known for triad connections and their staunch support for the pro-Beijing establishment.

Hong Kong's political crisis was spurred by an attempt to bundle through the legislature a Bill allowing extradition to China.

But protests have billowed into a wider pro-democracy movement, which has seen the financial centre's airport closed, violent street clashes with police and million-strong marches.

The city has enjoyed several peaceful protests recently - without police baton charges, tear gas or protesters hurling rocks. It is unclear in which direction the protests, which are leaderless and organised on social media, are heading after nearly three months of draining street action.

But the government has so far refused to give in to any of their demands, including scrapping the extradition Bill completely and a full independent inquiry into police actions during the protests.

Beijing also confirmed yesterday that an employee of Britain's consulate in Hong Kong who went missing earlier this month is being held in China.

Chinese Foreign Ministry spokesman Geng Shuang told a regular news briefing that the detained man had been "placed in administrative detention for 15 days as punishment" by Shenzhen police for breaking a public security law.

Mr Geng said the employee was from Hong Kong and therefore the issue was an internal matter.

"Let me clarify, this employee is a Hong Kong citizen, he is not a UK citizen, which is also saying he is a Chinese person," Mr Geng said.

The man, named by his family as Mr Simon Cheng, travelled to Shenzhen, a megacity on the China-Hong Kong border, for a one-day business meeting on Aug 8.

That night, Mr Cheng returned via high-speed train and sent messages to his girlfriend as he was about to go through Customs. "We lost contact with him since then," the family said in a Facebook post.

Mr Geng said the employee had violated the Public Security Administration Punishments Law - a law with broad scope aimed at "maintaining public order in society" and "safeguarding public security", as well as making sure police and security forces act within the law.

The ongoing protests have raised fears of a Chinese crackdown in some form.

Beijing has repeatedly warned Britain - the former colonial ruler of Hong Kong - against any interference in the protests, which erupted 11 weeks ago and have seen millions of people hit the streets calling for democratic reforms.

"Recently the UK has made many erroneous remarks about Hong Kong," Mr Geng said at the news briefing yesterday. "We once again urge the British side to stop gesticulating and fanning flames on the Hong Kong issue."

Friends of the missing employee staged a protest outside the British Consulate in Hong Kong yesterday afternoon to pressure the UK government to "save Simon".

"Simon is a very good guy, and smart guy... I don't think he would do anything stupid," said organiser Max Chung.