Prime Minister Stefan Lofven said everything indicates a "terror attack".
Swedish police say they have arrested one person in connection with Friday’s deadly truck attack in Stockholm.
At least four people were killed and 15 others wounded when a lorry rammed into crowds of shoppers in the centre of the city.
“One person has been arrested who may be connected with the incident,” police said in a statement.
Police had earlier released a grainy image of a man, caught on CCTV, saying he was a person of interest in the case.
Prime Minister Stefan Lofven said everything indicated the crash was “a terror attack”.
There was no immediate claim of responsibility.
What we know
Just before 3 pm local time, a truck ploughed into a crowd on Drottninggatan (Queen Street), one of the city’s main pedestrian streets, before crashing into a department store.
Part of central Stockholm was cordoned off and the area was evacuated, including the main train station. Large numbers of police and emergency services were at the scene. Parliament as well as the subway have been closed.
The lorry was hijacked earlier on Friday, its owner told local media. “During a delivery to the restaurant Caliente someone jumped into the driver’s cabin and drove off with the car while the driver unloads,” a spokesperson told the TT news agency.
“We were standing by the traffic lights at Drottninggatan (Queen Street) and then we heard some screaming and saw a truck coming,” a witness, who declined to be named, told Reuters.
“Then it drove into a pillar at Ahlens City (the department store) where the front started burning. When it stopped we saw a man lying under the tire. It was terrible to see,” said the man, who saw the incident from inside his car.
Police and the emergency services were quickly on the scene as video showed people running away and witnesses spoke of people lying motionless on the ground.
“Sweden has been attacked. Everything points to the fact that this is a terrorist attack,” Prime Minister Stefan Lofven told reporters during a visit in western Sweden. He immediately returned to the capital.
The police later showed a picture of a man wearing a grey hoodie and said they were looking for him, adding that they did not rule out the possibility that other attackers were involved.
Several attacks involving trucks or cars being driven into crowds have taken place in Europe over the past year. The number of dead from the most recent attack, in London on March 22, was raised to six on Friday with the news that a young Romanian tourist, Andreea Cristea, had died from her injuries.