Two people have been killed and at least six others wounded in a stabbing in the south-western Finnish city of Turku.
Police shot the suspected attacker and arrested him. They say they know his identity, but have not disclosed it.
One of the victims, a woman, was stabbed several times in the neck on Friday. The victims were adults and three are in intensive care.
Police have raided a flat and made a number of arrests overnight.
"We are investigating what their role is in this. Whether they had something to do with this act, or if they were just involved with this person," detective superintendent Markus Laine told Finnish news agency SST.
The raided flat is in the Varissuo area of Turku, home to a large immigrant population.
Earlier, Interior Minister Paula Risikko described the suspect as "foreign-looking" and linked the attack to the murder of 14 people in Barcelona.
Finnish authorities have said, however, that it is too early to describe the incident as terrorism.
The attack happened in the Puutori-Market Square area of Turku.
Police said the alarm was raised at 16:02 local time (13:02 GMT) on Friday, when a man was reported to have attacked passers-by with a knife. Witnesses said he then ran to a nearby square and stabbed more people before he was shot in the thigh. Police said he was arrested at 16:05.
Eyewitness Jesse Brown told the BBC: "I saw police shoot a person, a man I think. People were running and there was talk about a knife attack, possibly multiple perpetrators."
Later, a spokesman for Finland's National Bureau of Investigation said that while the authorities were trying to find out if there were others involved, it was likely that the suspect had been acting alone.
Another witness, student Anttoni Numminen, said he had been playing pool in a bar when he heard gunshots. People began rushing into the bar, he said, and "the place immediately went on lockdown".
Finland's Prime Minister Juha Sipila said his government was monitoring the situation closely. The interior minister and national police commissioner also said they were travelling to the city.
Security was also increased at key sites in the southern capital of Helsinki, where armed police could be seen at the central train station and the airport.
European Commission President Jean-Claude Juncker said that while details were still emerging "we strongly condemn this unprovoked attack which comes only 24 hours after the horror that unfolded in Spain".
Finland has no past experience of jihadist attacks but it has seen a number of fatal gun attacks:
- Three women were shot dead as they left a restaurant in south-east Finland in December 2016
- A trainee chef shot dead 10 people at a college in 2008
- A gunman killed eight people at a school in 2007