Suspected car-ramming in Sweden's capital brings panic to Stockholm
Swedish media have reported three people dead in the incident.
Swedish broadcaster SVT says shots have been fired and people in the area are fleeing the scene.
"Police received a call from SOS Alarm that a person in a vehicle has injured other people on Drottninggatan," police wrote on Twitter. The incident occurred just before 1pm near the city's biggest pedestrian street.
Thick smoke was rising from the scene, but police have yet to confirm a motive.
Witness Jan Granroth told local daily news outlet Aftonbladet "we stood inside a shoe store and heard something ... and then people started to scream. I looked out of the store and saw a big truck."
Video images showed an area of Stockholm sealed off by police and crowds gathering around the cordon.
Helicopters were hovering in the sky over central Stockholm, and a large number of police cars and ambulances were dispatched to the scene, according to witness reports.
It was not immediately clear whether it was an accident or an attack.
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Agencies contributed to this report