The gunman opened fire from the rooftop of a house in the southern Finnish town of Hyvinkaa early Saturday, Euronews reported.
Two 18-year-olds -- a man and a woman -- were shot and killed. Among the wounded were members of the Finnish baseball team, Hyvinkaan Tahko, and a policewoman.
Some of the injured are in serious condition, police said.
Police arrested an 18-year-old man wearing camouflage clothing several hours after the shooting, the BBC reported.
"The man was found with two weapons... including a hunting rifle," said Detective Chief Inspector Markku Tuominen, adding that he had no license for his weapons. The suspect offered no resistance to arrest, Tuominen said.
Police have not yet commented on a possible motive for the shooting.