At least four people were killed when a truck was driven into pedestrians on the busiest street in the center of Stockholm, Swedish police say.
Several people were also injured in the incident on Drottninggatan (Queen Street), one of the city's major pedestrian streets.
Swedish Prime Minister Stefan Lofven said everything pointed to an act of terrorism.
Local media reported one arrest, but police have said no-one is being held.
The truck was hijacked earlier on Friday, its owner said.
The crash happened at the Ahlens department store just before 15:00 local time.
Police in Stockholm have urged people to stay at home and avoid the city center. Parliament and the Stockholm subway were placed in lockdown. All train services in and out of Stockholm Central Station were halted and people were evacuated, the Swedish state railway company said.