Swedish police have arrested a man they suspect drove a beer delivery truck which rammed into a crowd in central Stockholm on Friday, killing four people and wounding 15 in what they described as a terror crime.

Police declined to comment on the identity or possible motive of the man, who they detained in a northern Stockholm suburb but Swedish public radio, citing unnamed sources, said he was from Uzbekistan.

"The person in question has been arrested as the culprit ... in this case the driver," police spokesman Lars Bystrom said on Saturday of the attack, adding that the authorities were not ruling out the possibility that he had accomplices, although only one person had been taken into custody.

There was no immediate claim of responsibility for Friday's attack, in which a hijacked truck was used, and police said security at Sweden's borders had been heightened and traffic was restricted on the Oresund Bridge linking Denmark and Sweden.

Vehicles have been used as weapons in Nice, Berlin and London in the past year in attacks claimed by Islamic State.

Police declined to comment on a report by public broadcaster SVT which said a bag containing a home-made bomb had been found in the truck. The report said the bomb may have partly exploded, burning the driver.

The beer truck, hijacked on Drottninggatan (Queen Street) in central Stockholm, ploughed through crowds before ramming into the Ahlens department store. The driver escaped in the chaos.

Local authorities in the capital, where flags flew at half mast on buildings including the parliament and royal palace, said that six of those injured had been able to leave hospital, while eight adults and one child remained in hospital.

The truck was removed overnight to be examined by forensics experts, leaving a gaping hole in the wall of the store. Dozens of people gathered at the site to pay their respects.

"Three minutes of terror and death," was the headline in daily tabloid newspaper Aftonbladet which carried a picture of an injured woman sitting in the street.

On Saturday morning, in a nearby open-air market, owners were returning to abandoned fruit and vegetable stalls after a defiant message from the country's prime minister.

"You will not defeat us, you will not govern our lives, you will never, ever win," Stefan Lofven, who described the assault as a terrorist attack, said late on Friday.

HIGH ALERT

The attack was the latest to hit the Nordic region after shootings in Danish capital Copenhagen in 2015 that killed three people and the 2011 bombing and shooting by far-right extremist Anders Behring Breivik that killed 77 people in Norway.

Although Sweden has not seen a large-scale attack, a failed suicide bombing in December 2010 killed the attacker only a few hundred yards from the site of Friday's incident.

Police in Norway's largest cities and at Oslo airport will carry weapons until further notice following the attack. Denmark has been on high alert since the February 2015 shootings.

Several attacks in which trucks or cars have driven into crowds have taken place in Europe in the past year.

Al Qaeda urged its followers to use trucks as a weapon in 2010 and Islamic State claimed responsibility for an attack in Nice, France, in July 2016, when a truck killed 86 people celebrating Bastille Day, and one in Berlin in December, when a truck smashed through a Christmas market, killing 12 people.

And last month, a man in London drove into pedestrians near Britain's parliament and then stabbed a policeman to death before being killed himself. Six people died in total.

"Our thoughts are going out to those that were affected, and to their families," Sweden's King Carl Gustaf said regarding the Stockholm attack, while European Union President Jean-Claude Juncker said an attack on any member state "is an attack on us all".

In February U.S. President Donald Trump falsely suggested there had been an immigration-related security incident in Sweden, to the bafflement of Swedes.

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.