Protesters face riot police inside the Yuen Long MTR station, the scene of an attack by suspected triad gang members a month ago, in Yuen Long, New Territories, Hong Kong, China August 21, 2019. Thousands of Hong Kong residents held a sometimes scrappy anti-government protest at a suburban subway station that was attacked by a mob last month, angry that nobody has yet been prosecuted for the violence.

REUTERS/Willy Kurniawan

Protesters face riot police during a stand off at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, August 21. Some masked protesters clashed with police, spraying fire extinguishers from the inside of Yuen Long station as others smeared the floor with cooking oil, beer and detergent to stop the police advancing.

REUTERS/Kai Pfaffenbach

Protesters fire nitrogen extinguishers toward riot police during a stand off at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, August 21. Some blocked station exits with bins, booths and other station furniture as others sealed roads outside the station, aiming green laser beams at the lines of shield-bearing officers. Others threw empty fire extinguishers at police lines. Many inside the station sat quietly.

REUTERS/Kai Pfaffenbach

Protesters face riot police inside the Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, August 21. The protest marked the night of July 21, when more than 100 white-shirted men stormed the Yuen Long station hours after protesters had marched through central Hong Kong and defaced China's Liaison Office - the main symbol of Beijing's authority. Using pipes and clubs, the men attacked black-clad protesters returning from Hong Kong island as well as passers-by and journalists, wounding 45 people.

REUTERS/Tyrone Siu

Protesters face riot police during a stand off at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21.

REUTERS/Kai Pfaffenbach

Protesters build a barricade inside Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Thomas Peter

Protesters gather after firing nitrogen extinguishers towards the riot police during a stand off at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, August 21. REUTERS/Kai Pfaffenbach

Police members remove protesters barricades during a stand off outside Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, August 21. REUTERS/Kai Pfaffenbach

Medics help a protester at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Ann Wang

Riot police asking the media to move at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Ann Wang

Protesters stand around a fare gate inside Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Ann Wang

A man gestures in front of police members at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Kai Pfaffenbach

Protesters stop the door of a Mass Transit Railway train at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Ann Wang

People walk inside Yuen Long MTR station after a stand-off with police in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Thomas Peter

Protesters beat the ceiling as they face riot police during a stand off at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Tyrone Siu

A protester stops the door of a Mass Transit Railway train at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Tyrone Siu

People protest during a silent sit-in gathering at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, August 21. REUTERS/Kai Pfaffenbach

People protest during a silent sit-in gathering at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, August 21. REUTERS/Tyrone Siu

Police members patrol as people protest at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, August 21. REUTERS/Willy Kurniawan

Protesters climb an escalator inside Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21, 2019. REUTERS/Ann Wang

Protesters stop the door of a Mass Transit Railway subway at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, August 21. REUTERS/Tyrone Siu

Police members are seen as people protest during a silent sit-in gathering outside Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, August 21. REUTERS/Kai Pfaffenbach

A protester gestures at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Ann Wang

Riot police members check the bag of a man at Yuen Long MTR station in Yuen Long, New Territories, Hong Kong, China August 21. REUTERS/Ann Wang