Police said they arrested one person after a shootout and had launched a manhunt for other possible attackers, according to Reuters.
The Turun Sanomat newspaper reported that at least one person was killed in the attack and that police were inspecting departing trains and buses.
Read alsoUkraine MFA condemns terrorist attack in BarcelonaAn eyewitness saw four or five ambulances near the city's main square, the paper said.
"The government is closely following the events in Turku and the ongoing police operation. The government will meet later today," Prime Minister Juha Sipila wrote on his Twitter account.
The incident comes a day after an attacker drove a van into a festive crowd at a highly popular tourist spot in Barcelona, Las Ramblas, killing 12 to 13 people, according to various reports, and leaving dozens injured.
Around 70 miles away in the seaside town of Cambrils, five terrorists wearing fake suicide belts were shot dead by police early on Friday after ramming pedestrians with a car in a second attack. One female civilian died and six other people, including a police officer, were injured, according to The Telegraph.
The Spanish terror suspects had been planning even bigger attacks, police have revealed as they continue a massive hunt for the rampaging van driver who plowed into a crowd in Barcelona.