A man steered a stolen beer truck into a crowd of people and then rammed it into a department store, killing at least three people in what officials were calling a terrorist attack in the heart of Stockholm on Friday afternoon.
“Sweden has been attacked. All indications are that it was a terrorist attack,” Prime Minister Stefan Lofven said in a statement.
The suspect in the attack was still at large. But at a news conference later in the evening, the Swedish police released a photo of a man being sought in connection for questioning.
The authorities said they did not know if it had been an isolated assault, or something bigger. The Swedish intelligence agency said “a large number” of people had been wounded.
Mats Löfving, the head of national operative department of the Swedish police, said, “This is now declared a national security event,” adding that officers across the nation were on heightened alert.
The Swedish Parliament was on lockdown, according to news reports. Train service in and out of the city grounded to a halt, and the police, who blocked off the affected area, urged people to stay at home and to avoid the city center.
The police said the first emergency call came in around 2:50 p.m. local time as the attack unfolded in Drottninggatan, Stockholm’s busiest shopping street. Witnesses described a scene of panic and terror.
“I saw hundreds of people running; they ran for their lives” before the truck crashed into the Ahlens department store, a witness identified only as Anna told the newspaper Aftonbladet.
Katarina Libert, a 32-year-old freelance journalist, was trying on clothes at the department store when she heard a boom and the walls shook.
At first, she said, she thought the noise was people moving things around the store, but then the fire alarm went off and staff members told her and other shoppers to get out of the building.
“We were running, we were crying, everyone was in shock,” Ms. Libert said. “We rushed down the street, and I glanced to the right and saw the truck. People were lying on the ground. They were not moving.”
Ms. Libert, who followed others as they were guided by officials to shelter, added, “My sister in law and some friends are close to the scene and at lockdown, can’t leave their office.”
She said that she usually avoided busy areas that could be potential terrorist targets, but that she had decided to take the Friday afternoon off to do some shopping.
“Some people felt that this was just a matter of time,” she said. “Paris, Brussels, London and now Stockholm. I just had a feeling something like this would happen.”
After the assailant plowed into people, the front of the truck ended up inside the department store.
A representative of the Spendrups brewery told Radio Sweden that the vehicle had been taken earlier in the day. A spokesman for the company told SVT, a national public broadcaster, that the truck had been stolen while the driver was loading it from the rear.
The brewery’s driver told the police that a masked man stole the vehicle, and that he was injured trying to stop him, the authorities said.
At the news conference, officials released a photo of a man wearing a hoodie. They did not name him as a suspect, saying only that they wanted to question him in connection with the attack.
The national police chief, Dan Eliasson, said, “We have the truck and the driver who usually drives it, but we do not have contact with the person or persons who drove it.”
Mr. Löfving, also of the police, asked for the public’s help in sharing the photo: “We want to get in touch with this man.”
The authorities also said that they could not confirm the number of dead or injured until they received more information from the hospitals.
The chief medical doctor at Stockholm’s Karolinska University Hospital, Nelson Follin, told the newspaper Dagens Nyheter that the hospital was treating “a handful” of people.
“The injuries are quite serious, but for now I cannot give further comments on conditions,” Dr. Follin said.
Previous accounts of shots being fired in parts of Stockholm were unfounded, the police said, adding that officers across Sweden were protecting high-risk sites.
The attack reverberated as far away as Norway, where the police said on Twitter that officers in that nation’s largest cities and at the airport in Oslo would be armed until further notice following the attack in Stockholm.
The assault came after several other episodes in Europe in the past year in which a vehicle was used to attack people.
The Islamic State group revived the idea of using cars as weapons after it broke with Al Qaeda in 2014. In the past year, ISIS militants have claimed responsibility for the deaths of more than 100 people in Europe.
In France, a man drove into a crowd on a busy seaside promenade during Bastille Day celebrations in Nice.
Another attacker plowed a truck into shoppers at a Christmas market in Berlin.
And last month, an assailant drove a car into pedestrians on Westminster Bridge near Parliament in London.
Other attempts, including an episode in which a man tried to drive over pedestrians in Antwerp, Belgium, claimed no victims, but have contributed to a sense of dread across the Continent.
Although some Swedes have expressed concern that immigration has led to a crime wave in the country — and President Trump seemed to suggest in a speech on Feb. 18 that there had been an attack in Sweden, when in fact nothing had occurred — the country and the region remain largely peaceful and safe.
The most notable exception came in 2010, when an assailant killed himself and wounded two others after detonating two bombs in central Stockholm, on a side street not far from where the attack on Friday took place.
The attack in 2010 was said to be the first suicide bombing in Scandinavia, and it caused consternation in Sweden. It was linked to an Iraqi-born Swede who had attended college in Britain.
On Friday, the police said they were well trained for these types of episodes. “Last week we rehearsed a similar scenario,” said Anders Thornberg, chief of national intelligence.