The shooting took place around 8.30pm at the entrance of a 7-Eleven convenience store near Odenplan on the city's north side.
"I heard three bangs that sounded like gunshots. There was a person lying on the ground and there was a lot of blood," one witness told the Aftonbladet newspaper.
Police patrols and an ambulance arrived shortly thereafter to find the victim conscious but seriously injured.
"The man was shot in the face. His injuries are serious and his condition has been described as critical," Norrmalm police commander Torgny Wallström told the TT news agency.
According to Aftonbladet, the man had been shopping at the 7-Eleven before the incident, and upon leaving the store was shot several times in the face.
In their search for the perpetrator, police closed down the Odenplan metro station in hopes of finding the shooter.
Later in the evening, a 26-year-old man was arrested on suspicion of attempted murder and taken in for questioning.
About an hour after the incident, police responded to calls about another shooting south of the city in Botkyrka.
On the scene they found an injured man who was taken to hospital for treatment.
Police have classified the shooting as attempted murder, but have yet to make any arrests in the case.
There are no indications that Tuesday's two shootings are related.