The injured 19-year-old, wearing black clothes and a black face mask, was knifed near one of the large “Lennon Walls” that have sprung up around the city during months of demonstrations, police said.
Local media images showed the man had been severely injured, with his inner organs visible where his abdomen had been cut in the afternoon incident in northeastern Tai Po district.
Footage posted on social media showed another man holding a knife shortly after the attack and shouting: “Hong Kong is part of China… (You) messed up Hong Kong”.
Police confirmed a 22-year-old man had been arrested as well as giving the age of the victim, who was conscious when he was rushed to hospital for surgery.
“The man suddenly rushed to my friend and slashed (him) in the neck. Then my friend ran away towards this direction. After that he fell down and was stabbed in the abdomen with a knife,” an associate of the injured man told media at the scene.
Plastered in colourful sticky-notes, posters and slogans, “Lennon Walls” have appeared in more than a hundred locations around Hong Kong, often in pedestrian tunnels or near subway stations.
Though the walls are seen as a peaceful protest method, they have also become flashpoints for violence.
In recent months, fights have broken out when groups of men who support the Hong Kong government have tried to tear the posters down, or between people with different political views.
On Wednesday Jimmy Sham — a leading face of Hong Kong’s pro-democracy movement — was taken to hospital covered in blood after being attacked with hammers by unidentified thugs.
The Civil Human Rights Front, which Sham leads, had applied for permission to hold a peaceful rally on Sunday calling for an independent inquiry into police brutality and universal suffrage, but their request was rejected by the police.
However the march is still expected to take place despite the ban.
Hong Kong’s more than four months of huge and increasingly violent protests were initially sparked by a now-scrapped bill that would have allowed extraditions to mainland China.
They have since snowballed into a wider movement calling for greater democracy and police accountability after Beijing and local leaders took a hard line.
Multiple pro-democracy activists have been attacked by pro-Beijing supporters in recent months, and Sham was also assaulted in August.
As the violence has escalated, hardcore pro-democracy protesters have also begun meting out their own street justice, beating people who vocally disagree with their goals or are viewed to be government loyalists.