Here is what we know about the truck attack in Sweden's capital:
The incident occurred just before 1300 GMT on Friday at the corner of the Ahlens department store and Drottninggatan, Stockholm's biggest pedestrian street.
"Police received a call from SOS Alarm that a person in a vehicle has injured other people on Drottninggatan," police wrote on Twitter.
Pictures showed a large blue truck with a mangled undercarriage smashed into the store.
Witnesses described scenes of panic and horror and authorities quickly sealed off the area.
Swedish authorities said four people had been killed and children were among the 15 others injured, nine of them seriously hurt.
Who was the attacker?
Prosecutors said police arrested a man 'for a terrorist crime' early Saturday. Police later said they believed he was the driver.
Police had earlier said they had detained a man who 'matched the description' of a suspect captured on video surveillance cameras near the scene of the attack.
The image showed a man wearing a white sweater and dark hoodie under a military green jacket, with dark stubble on his face.
Another man was arrested in Hjulsta, a working-class neighbourhood of Stockholm, but police refused to confirm a link with the attack.
Was It Terrorism?
Swedish Prime Minister Stefan Lofven described the incident as a 'terror attack'.
"Sweden has been attacked. Everything points to a terror attack," he said.
In 2010, another section of Drottninggatan was the scene of Sweden's only other terror attack, when a suicide bomber blew himself up, slightly injuring several others.
Friday's attack in Stockholm followed a string of similar assaults in Europe by people using vehicles as weapons.
Last month, Khalid Masood, a 52-year-old known to British security services, drove a car at high speed into pedestrians on London's Westminster Bridge before launching a frenzied knife attack on a policeman guarding the parliament building.
The incident killed five people, while Masood himself was shot dead by police.
How Did Authorities React?
Helicopters could be heard hovering in the sky over central Stockholm and a large number of police cars and ambulances were dispatched to the scene.
The centre of the usually buzzing city was on lockdown, with the central train station evacuated and other stores quickly emptied of shoppers.
Police vans circulating in the city using loudspeakers urged people to go straight home and avoid large crowds.
The Stockholm metro was also completely shut down for several hours before resuming in the early evening, with the attack taking place close to the city's T-Centralen station, through which all the city's lines pass.
Large public buildings were evacuated and closed down, such as shopping malls and cinemas, while parliament was on lockdown for several hours. Armed guards patrolled outside government offices.
What was the Reaction Abroad?
European politicians expressed solidarity, with European Commission President Jean-Claude Juncker saying the incident was an 'attack on us all'.
A spokesman for German Chancellor Angela Merkel said, "Our thoughts go out to the people in Stockholm," adding: "We stand together against terror."
French President Francois Hollande voiced his "horror and indignation" over the assault.
US State Department acting spokesman Mark Toner condemned "this brutal and senseless attack" and said the United States was ready to provide any assistance it could to investigate it.
UN Secretary General Antonio Guterres said, "We hope that those responsible for the attack will be swiftly brought to justice".
Russian President Vladimir Putin said "Russians weep with the Swedish people".