A man arrested in Stockholm after Friday's fatal truck attack is believed to be the driver, Swedish police say
The man, who was arrested in the north of the city, has not been named, but Swedish media say he is from Uzbekistan.
Four people were killed and 15 were injured when the truck crashed into the front of a department store.
Police said they were unable to confirm local television reports of explosives being found inside the truck.
The truck - hijacked from outside a nearby restaurant - crashed into the front of Ahlens department store in the middle of the afternoon on Drottninggatan (Queen Street), one of the city's main pedestrian thoroughfares.
Eyewitnesses described horrifying scenes outside the store, as bodies and injured people lay on the ground.
Swedish Prime Minister Stefan Lofven has called it a terrorist attack, with borders tightened at his request.
The crash happened at the Ahlens department store just before 3pm local time, with witnesses saying the truck drove directly into the front windows of the store.
Swedish brewery Spendrups said its truck had been stolen earlier on Friday on its way to a restaurant delivery.
"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.
Central Stockholm was in lockdown after the incident.
In the immediate aftermath, shoppers took refuge in nearby buildings for hours, waiting for the area to be secured.
The central station, which was near the Ahlens store, was evacuated.
One eyewitness told the BBC she was in the shop's fitting room when she heard the screams. "There was blood everywhere," she said.
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.
City authorities said they had made several spaces, including a number of school buildings, available as temporary accommodation for those who cannot get home because of transport disruptions.
They have also asked people not to share photographs of the victims online.
Vehicle ramming attacks in Western countries
- 14 July 2016, Nice, France: A man drove a truck 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
- 19 December 2016, Berlin, Germany: The attack in Berlin killed 12 people and injured 49, when a man drove a truck 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
- 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
The Sapo security police said in its annual report it was impossible to say how big a risk there was that Sweden would be targeted like other European cities, but that, if so "it is most likely that it would be undertaken by a lone attacker".
Sweden has not previously been hit by a large-scale terror attack, although in December 2010 a man blew himself up only a few hundred yards from the site of the latest incident in a failed suicide attack.
In February, US President Donald Trump falsely suggested there had been an immigration-related security incident in Sweden, to the bafflement of Swedes.
Swedish authorities raised the national security threat level to four on a scale of five in October 2010 but lowered the level to three, indicating a "raised threat", in March 2016.
Police in Norway's largest cities and at Oslo's airport will carry weapons until further notice following the attack. Denmark has been on high alert since the February 2015 shootings.
Neutral Sweden has not fought a war in more than 200 years, but its military has taken part in U.N peacekeeping missions in a number of conflict zones in recent years, including Iraq, Mali and Afghanistan.