“Sweden has been attacked,” Lofven said. “This indicates that it is an act of terror.”
Lofven made the comment Friday after the truck crashed into the upscale Ahlens department store on Drottninggatan Street.
He also said one person has been arrested, but no further details were immediately available.
The number of people reportedly killed in the attack varies.
Lofven said at least two were killed, Broadcaster SVT says at least five, while Swedish radio says least three.
Police could not immediately confirm the death toll.
Live television footage on Friday showed smoke coming out of the store, which is part of a Sweden-wide chain. The building housing the Stockholm store includes several stores at street-level.
“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.”
The Aftonbladet daily said Swedish beermaker Spendrups reported its truck had been carjacked earlier Friday.
The most recent attack in Stockholm was on Dec. 11, 2010, when an Iraqi-born Swede, Taimour Abdulwahab al-Abdaly, detonated two devices, including one that killed him, in central Stockholm.