Eyewitness accounts in Finnish media reports said the assailant burst into a classroom in Savo Vocational College in the town of Kuopio on Tuesday morning.
The attacker was also injured and was taken into custody, according to police, Presstv Reported.
“Officers used firearms during the situation. Police have detained one perpetrator. The injured have been evacuated,” East Finland police said in a statement.
An unnamed eyewitness told Keskisuomalainen newspaper that the assailant “hit a girl in the neck with a sword and stabbed her in the stomach” and also set off “some sort of small firebombs.”
Another eyewitness, Roosa Kokkonen, who works in a car garage opposite the college, told Finnish TV channel MTV that a teacher with blood running from her hand came fleeing out of the building.
“While I was helping the teacher, I started hearing other shouts for help. Students were running away and into my garage,” Kokkonen said.
She also told Finnish news agency STT that students described the weapon as “a long sword,” and that the assailant “started swinging the sword around in the class.”
Police said two of the injured were in serious condition.
In a tweet, Finnish Prime Minister Antti Rinne described the attack as “shocking and utterly reprehensible.”
Finland is a relatively safe country, with a total of 73 homicides in 2017. It has a large gun-owning population.
The Nordic country has seen two deadly school attacks in the past 12 years. In 2007, a student at the Jokela High School shot eight people dead before killing himself. In 2008, a 22-year-old student at a vocational school in Kauhajoki shot 10 dead before committing suicide.