Swedish radio said Friday that three people have been killed in the crash and Swedish broadcaster SVT said shots have been fired. People in the area were seen fleeing the scene. Swedish police has ordered the evacuation of the central train station.
The Swedish news agency TT said several people have been rushed away in ambulances, and live television footage showed smoke coming out of the department store that the truck smashed into.
Swedish police showed a picture of a person they said was of interest.
Mats Lofving, head of Swedish police's National Operations Department (NOA) said the picture, which appeared to be CCTV footage, was taken close to the time of the incident, in the vicinity of the incident.
"I have a picture of a person who has been seen at the location at this point in time. We want to get in contact with this person," he told a news conference.
The grainy image shown by police showed a man wearing a jacket with a dark hood over a bright t-shirt and dark trousers.
Swedish police said they have received calls about a person who has injured others driving a vehicle on the central Stockholm street of Drottninggatan. Police spokeswoman Towe Hagg said people have been injured but she would not confirm the deaths.
Swedish police issued a warning to avoid the city center.
Dimitris, an eyewitness to the event, told the Swedish newspaper Aftonbladet: "I was walking down the main street and suddenly a big truck came out of nowhere, I couldn’t see who was driving it, but it got out of control. I ran away as fast as I could."
Friday's incident is near the site of a December 2010 attack in which Taimour Abdulwahab, a Swedish citizen who lived in Britain, detonated a suicide bomb, killing himself and injuring two others.
Abdulwahab rigged a car with explosives in the hope that the blast would drive people to Drottninggatan—the street hit Friday—where he would set off devices strapped to his chest and back. The car bomb never went off, and Abdulwahab died when one of his devices exploded among panicked Christmas shoppers.
There was no immediate claim of responsibility for Friday's attack, but vehicles have been common weapons in recent extremist attacks. Last month, in an attack claimed by the Islamic State group, a man drove into a crowd on London's Westminster Bridge, killing three people and injuring many others before stabbing a policeman to death. A fourth person, a woman thrown into the Thames by the force of the car, died Thursday.
The incident in Sweden came months after the terrorist attack in Berlin in which an Islamist terrorist committed a truck-ramming attack, claiming 12 lives, including an Israeli citizen, and wounding dozens.
In a similar attack, just over six months ago, an ISIS terrorist who drove a truck ran over 85 people and injured more than 100 during France's Independence Day celebrations in Nice.