Swedish state radio reported that three had died in the incident.
Swedish Prime Minister Stefan Lofven said the country has been attacked and that everything indicated that the truck driven into a crowd in central Stockholm was an act of terrorism, news agency TT reported.
An area of central Stockholm was cordoned off and large numbers of police and emergency services were at the scene. The area was evacuated.
“I saw at least three dead , but probably more,” Swedish radio reporter Martin Svenningsen said.
A witness saw a number of body-like forms covered by blankets at the scene of the incident. Thick smoke was rising from the scene, while video images showed an area blocked off by police and crowds gathering around the police cordon.