Swedish police were hunting a terror suspect who hijacked a truck and ploughed into a shopping centre in Stockholm killing at least four people and injuring 15.
Emergency services said four people died and nine of the 15 injured, including children, were in a serious condition in hospital.
Police said the driver of the truck had fled and issued an image of a man they wanted to trace in connection with the attack at the busy shopping district.
The image showed a man dressed in a grey hooded jacket stepping from an escalator.
One person was arrested later in the day who matched the description of a suspect they were looking for, police said.
A spokesman initially said there had been no arrests, adding: “For the moment police are searching for a person or persons."
Officers also confirmed they were interviewing two people in relation to the truck attack but that "it did not mean they were suspects".
Prime Minister Stefan Lofven said the incident which began at about 3pm local time appeared to have been "a terror attack".
He told reporters during a visit in western Sweden: "Sweden has been attacked. Everything points to the fact that this is a terrorist attack." He was immediately returning to the capital.
The truck had been stolen while making a delivery to a restaurant further up Drottninggatan, Spendrups Brewery spokesman Marten Lyth said. A masked person jumped into the cab, started the truck and drove away, he said.
Witnesses described how they "ran for their lives" and hid in terror after the attack began.
They told how victims lay dead in the street while medics and members of the public treated the injured.
Glen Foran, an Australian tourist in his 40s, said: "I turned around and saw a big truck coming towards me. It swerved from side to side. It didn't look out of control, it was trying to hit people,"
"It hit people, it was terrible."
Another witness, who asked not to be named, said: "We were standing by the traffic lights at Drottninggatan and then we heard some screaming and saw a truck coming.
"Then it drove into a pillar at Ahlens City [department store] where the hood started burning. When it stopped we saw a man lying under the tyre. It was terrible to see."
Dramatic images showed a huge police cordon at the scene and a swarm of emergency services vehicles in the busy shopping district.
Several people were seen being rushed away in ambulances, and live television footage showed smoke pouring out of the department store.
At a press conference national police commissioner Dan Eliasson said: "A truck was driven into Ahlen's department store on Drottninggatan. There are many people injured."
He added: "We have no contact with the person or persons who drove the truck."
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."
All trains to and from the main station were halted and two large shopping malls in the capital were shut down after the attack.