Several people have reportedly died and many have been left injured after a hijacked truck crashed into crowds of people before ramming into a department store in central Stockholm.
Gunshots were also reportedly heard as panicked shoppers fled during the deadly rampage on Drottninggatan (Queen Street), which began just before 3pm local time.
Swedish Prime Minister Stefan Lofven said at this stage everything indicated it was a terror attack.
"Sweden has been attacked. Everything points to the fact that this is a terrorist attack," Mr Lofven said at a press conference.
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Police confirmed three deaths and eight people injured, however the media have reported varying death tolls.
No one has been arrested in connection with the attack, police said, and there has been no immediate claim of responsibility.
Part of central Stockholm has subsequently been cordoned off and the area evacuated, including the main train station. All subway traffic was halted on orders from the police.
The Swedish news agency TT said several people had been rushed away from the scene in ambulances, and live television footage showed smoke coming out of the department store that the truck smashed into.
The news agency also reported that the Swedish brewery company Spendrups, which owns the truck used in the attack, said the vehicle had been hijacked earlier in the day.
"During a delivery to the restaurant Caliente someone jumped into the driver's cabin and drove off with the car while the driver unloads," communications director Mårten Lyth said.
One eyewitness, identified only as Durde, told the Aftonbladet daily the truck was driving at full speed when it ploughed through crowds of shoppers, while another bystander, Dimitris, described the vehicle as being "out of control".
A government source told Reuters all Swedish government offices had since been closed. Stockholm Police have urged the public to stay at home and avoid the city.
This is a breaking news story. More detail will be added shortly.