A 39-year-old Uzbek national arrested on "suspicion of terrorist crime" was likely to be the driver of a truck that rammed into a crowd of people in central Stockholm a day earlier, killing four, according to Swedish authorities.

Sweden's police chief said on Saturday authorities were confident they had detained the man who carried out the attack.

"There is nothing that tells us that we have the wrong person," Dan Eliason said, but added he did not know whether others were involved in the attack. "We cannot exclude this."

Swedish media reported police had arrested a second man and that he had a connection to the previously arrested person, citing police sources. The police declined to comment on whether it had arrested any additional suspects.

Eliason also said police found something in the truck that "could be a bomb or an incendiary object, we are still investigating it".

Prosecutor Hans Ihrman said the suspect has not yet spoken to authorities and could not confirm whether he was a legal resident of Sweden.

READ MORE: Stockholm: Beer truck ploughs into department store

The incident, which also left 15 people injured, occurred just before 13:00 GMT on Friday at the corner of the Ahlens department store and Drottninggatan, the Swedish capital's biggest pedestrian street, above-ground from its central metro station.

Sweden's Prime Minister Stefan Lofven said everything indicated it was "a terror attack".

"We are thinking of the dead and the injured and their families. I am urging the public to be vigilant and keep updating the police," Lofven said.

The capital was paralysed with a huge security presence after the attack. Police told people to stay away from the centre of the city, and movie theatres, as well as many shops, were closed.

The metro system was halted, train traffic from the nearby Central Station disrupted and thousands of people were seen walking across the city's bridges with no alternative transportation available.

Photos from the scene showed a beer truck sticking out of the department store.

Swedish beer maker said one of its vehicles had been carjacked earlier on Friday as its driver was unloading goods at a restaurant.

"We stood inside a shoe store and heard something ... and then people started to scream," witness Jan Granroth told the Aftonbladet daily.

"I looked out of the store and saw a big truck."

Body-like forms covered by blankets were seen on Drottninggatan.

The vehicle caught fire after driving through the busy pedestrian zone and slamming into the building.

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The European Union offered Sweden support and solidarity on Friday.

"An attack on any of our member states is an attack on us all," said EU chief executive Jean-Claude Juncker.

"One of Europe's most vibrant and colourful cities appears to have been struck by those wishing it - and our very way of life - harm."

A spokesman for German Chancellor Angela Merkel said: "We stand together against terror."

Friday's incident was 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.

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

In London last month, a man in a car drove into pedestrians on Westminster Bridge, killing four, and then stabbed a policeman to death before being shot by police.

In Nice, France, last July, a truck killed 86 people celebrating Bastille Day, and one in Berlin in December smashed through a Christmas market, killing 12 people.