Media playback is unsupported on your device Media caption Footage shows people fleeing Stockholm truck crash scene
A lorry has smashed into a store in central Stockholm, killing at least four people.
At least a dozen people were also injured in the incident on Drottninggatan (Queen Street), one of the city's major pedestrian streets, on Friday afternoon.
Swedish Prime Minister Stefan Lofven said everything pointed to an act of terrorism.
Police said they were still hunting for the driver.
A different person, who matched the description of a suspect, was arrested earlier after police released a grainy image caught on CCTV.
He is deemed to be a person of interest in the case.
Eyewitnesses describe lorry 'trying to hit people'
Image copyright Swedish police handout Image caption Police wanted to speak to this man - a suspect matching his description was later arrested
In a speech on Friday night, Mr Lofven said Swedish values of democracy and freedom would not be "undermined by hatred".
He also announced the country's border controls had been strengthened.
Media playback is unsupported on your device Media caption PM Stefan Lofven says the "values we treasure" will not be affected by the attack
The crash happened at the Ahlens department store just before 15:00 local time (13:00 GMT). Witnesses say the lorry drove into the front window.
One eyewitness, Annevi Petersson, told the BBC she was in the shop's fitting room when she heard the screams. "There was blood everywhere," she said.
Image copyright Reuters Image caption The lorry crashed into the Ahlens department store in the city's centre
Swedish brewery Spendrups said its lorry had been stolen on its way to a restaurant delivery earlier in the day.
"Someone jumped into the driver's cabin and drove off with the vehicle while the driver was unloading," a brewery spokesperson told the TT news agency.
The shop sits close to the city's central station, which was evacuated.
The metro, central roads and various bus lines were also shut down after the attack.
Media playback is unsupported on your device Media caption Eyewitness: 'I heard the screams'
Shots were reportedly fired in another part of the city, but Swedish police told local media there was no connection between the two incidents.
City authorities said they had made several spaces, including a number of school buildings, available as temporary accommodation for those who could not get home because of transport disruptions.
They have also asked people not to share photographs of the victims online.
Analysis, by Frank Gardner, BBC's Security Correspondent
I was in Stockholm yesterday, ironically at a security conference. I don't think Sweden was prepared for something like this.
The last big terror incident they had was in 2010 when a failed suicide bomber blew himself up in a car in central Stockholm.
The country has good intelligence-gathering apparatus: Sapo, the Swedish equivalent to the UK's MI5 agency, have got the finger on the problem, but the legislation is not there to deal with it.
Insiders have been concerned for quite some time that Sweden has been a bit lax about security. It is important at this stage, though, to keep an open mind about the perpetrator. It's not impossible that it was someone with personal, psychological problems, rather than a political motive.
Timeline: Vehicle ramming attacks in the west
14 July 2016, Nice, France : A man drove a lorry for 2km (1.2 miles) through a large crowd gathered to watch Bastille Day fireworks in Nice. Eighty-six people were killed, and more than 300 injured.
A man drove a lorry for 2km (1.2 miles) through a large crowd gathered to watch Bastille Day fireworks in Nice. Eighty-six people were killed, and more than 300 injured. 28 November 2016, Ohio, United States : An 18-year-old student rammed his car into a group of pedestrians at Ohio State University and stabbed others. Eleven people were injured before he was shot and killed.
An 18-year-old student rammed his car into a group of pedestrians at Ohio State University and stabbed others. Eleven people were injured before he was shot and killed. 19 December 2016, Berlin, Germany : The attack in Berlin killed 12 people and injured 49, when a man drove a lorry through the crowded Breitscheidplatz Christmas market. So-called Islamic State said one of its "soldiers" carried out the attack.
The attack in Berlin killed 12 people and injured 49, when a man drove a lorry through the crowded Breitscheidplatz Christmas market. So-called Islamic State said one of its "soldiers" carried out the attack. 22 March 2017, London, United Kingdom : Six people died and at least 50 were injured when a car mounted the pavement on London's Westminster bridge and drove at high speed through pedestrians. The attacker then entered the parliament complex on foot and fatally stabbed a police officer, before being shot.
Six people died and at least 50 were injured when a car mounted the pavement on London's Westminster bridge and drove at high speed through pedestrians. The attacker then entered the parliament complex on foot and fatally stabbed a police officer, before being shot. 23 March 2017, Antwerp, Belgium: A man was caught by soldiers after he drove at a crowd. Knives, a non-lethal gun and a dangerous substance were found in his car - but no-one was injured. Terror charges were later dropped.
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SmokingBerra on April 7th, 2017 at 13:46 UTC »
The company owning the truck says it was stolen during a delivery to a restaurant, according to Swedish newspaper Aftonbladet
Deslan on April 7th, 2017 at 13:44 UTC »
Radio P4 reporter says that he himself saw 3 dead. Truck is on fire. House that truck ran into is also taking on fire with increasing smoke development. Fire department working on it. Area around the truck is closed off. People are lying on the street. Security guards, not police, are forcing people away from the accident area, telling people to avoid groups. Next to the truck was a hot dog stand. One block away people are carrying on as if nothing happened. They probably don't even know about it. Increased police activity all over town but no signs of anything else happening. Police is warning about possible terrorist attacks being carried out. radio reporter now says at least 5 dead. He helped with first aid, unsuccessful. Site is Drottninggatan next to Hötorget, store name is Åhléns. Witness says it's a beer truck. Government building had been sealed off. Riot police are setting up stations. EDIT: truck was recently stolen from around the corner of the accident. All subways are closed off.
desyncg on April 7th, 2017 at 13:43 UTC »
I just want to add that Drottninggatan is a huge shopping street and essentially a highway for pedestrians.