Police are questioning a man after a lorry was used to mow down pedestrians, killing four, as terror struck in the heart of Stockholm.
Children were among the 15 injured, emergency services said, when the vehicle brought carnage to a busy shopping district in the Swedish capital.
Nine victims were left in a serious condition and reports suggested a pram was caught in the path of destruction.
A nationwide manhunt was launched and Swedish police said one man was arrested in the Stockholm area.
A Stockholm police spokesman said on Friday night that others are still being sought while officers try to confirm whether the man in custody is the driver.
"We are looking for more people. We are not quite sure if the person we have arrested is the driver," he said.
"We can't confirm that at the moment."
The latest outrage inflicted on the continent came just two weeks after similar tactics were used to attack London, as a car drove into crowds on Westminster Bridge.
Swedish security services said they were treating the atrocity as a terror attack, but police added they had yet to confirm it as such.
Detectives arrested the man after issuing a picture of an individual they wished to speak to, later saying he matched the description of the image.
Border control has been reinforced following the attack and the area where the lorry crashed remains sealed off, police said.
Condemnation poured in as news of the attack broke, including statements from German chancellor Angela Merkel and Foreign Secretary Boris Johnson, who said he was "deeply concerned".
EU Commission president Jean-Claude Juncker said: "An attack on any of our member states is an attack on us all."
As evening fell, Swedish prime minister Stefan Lofven laid a bouquet of red roses near the site of the attack, lighting a candle.
The lorry smashed into the corner of a department store and burst into flames.
Senior police officer Mats Lofving said: "We don't know whether this incident is isolated or whether we can expect more.
"We have police positioned at several strategic places with a particular risk threat."
The bloodshed bore hallmarks to attacks seen in Nice and Berlin last year, where a truck rammed pedestrians, and Muslim convert Khalid Masood's attack on Westminster Bridge in London.
The Aftonbladet newspaper reported that the truck had been hijacked from Swedish beer maker Spendrups earlier on Friday.
The lorry smashed through stone lion bollards into the pedestrianised area, leaving a trail of blood and debris.
Witness Jan Granroth told Aftonbladet that "we stood inside a shoe store and heard something ... and then people started to scream".
He said: "I looked out of the store and saw a big truck."
Another witness quoted by the paper said: "When I came out, I saw a lorry standing there with smoke coming from it and there were loads of bits of cars and broken flower pots along the street."
Mikael Anttila, a 49-year-old portfolio manager at bank SEB, told the Press Association he saw several hundred people gathered on the street close to the shop before they all started running "suddenly ... like ants".
"Then a lot of police started coming. Heavy weapons, civilian police, etc," he said.
Annevi Peterson described people lying dead and injured in the street, with blood everywhere.
"I heard the noise, I heard the screams, I saw the people," she told BBC News.
"There was, just outside the store, there was a dead dog, the owner screaming, there was a lady lying with a severed foot.
"There was blood everywhere, there were bodies on the ground everywhere.
"There was a sense of panic, people standing by their loved ones, but also people running away."
The crash is close to the scene of a terror attack in 2010 when Taimour Abdulwahab, a Swedish citizen who lived in Luton, blew himself up.
The terrorist, who police concluded acted alone, died on December 11 2010 in the Bryggargatan area. No one else was killed.
Abdulwahab rigged an Audi car with explosives in the hope that the blast would drive people to Drottninggatan, a busy shopping street, where he was waiting to set off two more devices strapped to his chest and back.
The car bomb never went off and after setting fire to the Audi, he was unable to detonate the other two explosives as planned.
He made his way down a side street off Drottninggatan and, in an apparent attempt to fix the faulty trigger up his sleeve, set off the bomb on the front of his body, killing only himself.