Police in Hong Kong fired tear gas and rubber bullets at protesters defying official warnings not to march in a neighbourhood where, six days earlier, a mob had brutally attacked people in a train station.

Key points: Protesters streamed through Yuen Long, even though police refused to grant permission for the march

Protesters streamed through Yuen Long, even though police refused to grant permission for the march Three hours after the march started, police fired tear gas to disperse the crowd

Three hours after the march started, police fired tear gas to disperse the crowd Protesters chanted "Hong Kong police know the law and break the law"

Protesters wearing black streamed through Yuen Long, even though police had refused to grant permission for the march citing risks of confrontations between demonstrators and local residents.

For the protesters, it was a show of defiance against the white-clad assailants who beat dozens of people last Sunday night, including some demonstrators heading home after the latest mass protest in the summer-long pro-democracy movement.

Police said some of the attackers at the train station were connected to triad gangs and others were villagers who live in the area.

The streets of Yuen Long became a sea of umbrellas as the march began on Saturday afternoon.

A symbol going back to the Occupy Central protests that shook Hong Kong in 2014, umbrellas have become tools to help protesters conceal their identities from police cameras as well as shields against tear gas and pepper spray.

Some also wore masks to obscure their faces.

The Government warned protesters that police would move in to break up the demonstration. ( Reuters: Edgar Su )

"Hong Kong police know the law and break the law," protesters chanted as they made their way through the streets.

Hong Kong's public transit network had announced that its trains would not be making their usual stops in Yuen Long on Saturday.

Several businesses and public facilities were closed in anticipation of the march. Service at a nursing centre was temporarily suspended, and sports venues shut down early.

Violence escalates as protesters built barricades

Demonstrators used shields and umbrellas to protect themselves during the protests. ( Reuters: Edgar Su )

A few hours before the march started, a man was arrested in Yuen Long for injuring someone with a knife, police said.

Less than three hours after the start of the march, police fired tear gas to try to disperse the crowd.

The government issued a statement at around the same time warning that police would move in to break up the demonstration.

Police officers fired tear gas at demonstrators during the protest. ( Reuters: Tyrone Siu )

The statement said some protesters were "holding iron poles, self-made shields and even removing fences from roads," and that some had surrounded and vandalised a police vehicle with officers inside.

Earlier, the government said in another statement that police were concerned about a "possible deterioration of the situation."

"Police appeal to members of the public to stay calm and leave the area as soon as possible as a chaotic scene may ensue within a short period of time," the earlier statement said.

The violence escalated as protesters built barricades out of street furniture and umbrellas, throwing rocks and bottles.

Many armed themselves with hiking sticks and improvised shields from wood, surfboards, cardboard and other materials.

Police, widely criticised for failing to better protect the public from last weekend's attack, responded with tear gas, rubber bullets and sponge grenades, a crowd control weapon.

Police wearing heavy-duty helmets and wielding batons forcefully removed protesters from the closed Yuen Long train station after a few hundred people took refuge there from the tear gas.

Police forcefully removed protesters who took shelter in Yuen Long train station. ( AP: Eric Tsang )

Some officers swung their batons at protesters, prompting them to scream and run.

Earlier, witnesses saw a hard core group of activists with small metal bats, metal and wooden poles and slingshots moving against the human tide.

Several hundred protesters remained as dark fell, fighting with police in the train station, where blood could be seen spattered on the floor.

Demands grow after extradition bill backdown

Protesters labelled the demonstration a "shopping trip" to counter police objections. ( Reuters: Edgar Su )

Massive demonstrations began in Hong Kong early last month against an extradition bill that would have allowed suspects to face trial in mainland China, where critics say their rights would be compromised.

The bill was eventually suspended, but protesters' demands have grown to include direct elections, the dissolution of the current legislature and an investigation into alleged police brutality in the Chinese territory.

A former British colony, Hong Kong was returned to China in 1997 under the framework of "one country, two systems".

The arrangement promises the city certain democratic freedoms that are not afforded mainland citizens, but some residents say these liberties have been steadily eroded in recent years after the arrests of booksellers and democracy activists.

A distrust of China's Communist Party-led central government in Beijing has bolstered the protests this summer.

After last Sunday's march, a group of protesters vandalised Hong Kong's Liaison Office, which represents the mainland government.

They spray-painted the building's surveillance cameras and threw eggs and black ink at the Chinese national emblem, an act that Beijing has vehemently condemned.

In response to the police's objection to Saturday's march in Yuen Long, protesters have cheekily labelled the procession a "shopping trip," as well as a memorial service for former Chinese premier Li Peng, who died on Monday.

Li was a hard-liner best known for announcing martial law during the 1989 Tiananmen Square protests that ended in bloodshed.

A demonstrator is seen injured during the protest against the Yuen Long attacks. ( Reuters: Tyrone Siu )

Some Yuen Long residents participated in the march, while others stood outside with signs warning protesters not to enter. Demonstrators pasted calls for democracy on sticky notes around the area.

"After the violence (last Sunday), as a resident of Yuen Long, I think I have the responsibility to come out," said a 24-year-old man surnamed Man.

"After all these protests in past months, the government still hasn't responded to us."

AP