Stockholm (AFP) - A hijacked truck has slammed into a crowd of people outside a busy department store in central Stockholm, causing an unspecified number of "deaths" in what the prime minister described as a "terror attack."

Here is what we know about the truck attack in Sweden's capital:

- What happened? -

The incident occurred just before 1300 GMT on Friday at the corner of the Ahlens department store and Drottninggatan, Stockholm's biggest pedestrian street.

"Police received a call from SOS Alarm that a person in a vehicle has injured other people on Drottninggatan," police wrote on Twitter.

Pictures showed a large blue truck with a mangled undercarriage smashed into the store.

Witnesses described scenes of panic and horror and authorities quickly sealed off the area.

Swedish authorities said they could not immediately provide a death toll or say how many people were injured but local media said two or three people had lost their lives in the attack.

"There are deaths, and many injured," Nina Odermalm Schei, a spokeswoman for Swedish intelligence agency Sapo, told AFP, providing no further details.

- Who was the attacker? -

Police on Friday released a grainy picture of a suspect captured on video surveillance cameras near the scene of the attack, but said they did not currently have him in custody.

"We do not have contact with the driver," national police chief Dan Eliasson told reporters.

The picture showed a man wearing a white sweater and dark hoodie under a military green jacket, with dark stubble on his face.

- Was it terrorism? -

Swedish Prime Minister Stefan Lofven described the incident as a "terror attack".

"Sweden has been attacked. Everything points to a terror attack," he said.

In 2010, another section of Drottninggatan was also the scene of Sweden's only other terror attack, when a suicide bomber blew himself up, slightly injuring several others.

Friday's attack in Stockholm followed a string of similar massacres in Europe by people using vehicles as weapons.

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The deadliest came last year in France on the Bastille Day national holiday of July 14, when a man rammed a truck into a crowd in the Mediterranean resort of Nice, killing 86 people.

Last month, Khalid Masood, a 52-year-old convert to Islam known to British security services, drove a car at high speed into pedestrians on London's Westminster Bridge before launching a frenzied knife attack on a policeman guarding the parliament building.

The incident killed five people, while Masood himself was shot dead by police.

- How did authorities react? -

Helicopters could be heard hovering in the sky over central Stockholm and a large number of police cars and ambulances were dispatched to the scene.

The centre of the usually buzzing city was in lockdown, with the central train station evacuated and other stores quickly emptied of shoppers.

Police vans circulating in the city using loudspeakers urged people to go straight home and avoid large crowds.

The Stockholm metro was also completely shut down, with the attack taking place at the city's T-Centralen station, through which all the city's lines pass.

Large public buildings were evacuated and closed down, such as shopping malls and cinemas, while parliament was on lockdown for several hours. Armed guards patrolled outside the government offices.

- What was the reaction abroad? -

European politicians expressed solidarity, with European Commission President Jean-Claude Juncker saying the incident was an "attack on us all."

A spokesman for German Chancellor Angela Merkel said: "Our thoughts go out to the people in Stockholm," adding: "We stand together against terror."

French President Francois Hollande voiced his "horror and indignation" over the assault.

UN Secretary General Antonio Guterres said: "We hope that those responsible for the attack will be swiftly brought to justice".