Swedish police found a homemade explosive device in the stolen truck that was used in Friday's deadly terror attack in central Stockholm, Swedish television SVT reports citing unnamed sources.

SVT says the device was in a bag, but did not explode.

Police spokesmen Lars Bystrom declined to confirm the report to dpa, saying "we never comment on what we seize."

Swedish public broadcaster SVT said that police had found a bag containing explosives.Four people were killed and at least 15 injured when a stolen truck slammed into a crowd of people outside a busy department store in central Stockholm, in what the prime minister described as a "terror attack."

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 four people had been killed and children were among the 15 others injured, nine of them seriously hurt.