Shots have also reportedly been fired, the BBC reported.
Swedish police said a number of people were injured.
The incident took place on Drottninggatan (Queen Street), one of the city's major pedestrian streets, just before 15:00 local time (14:00 GMT).
Something's happening on Drottninggatan and around Stockholm. pic.twitter.com/YPbrZSe5Mb— Johnny Chadda (@johnnychadda) April 7, 2017
Witnesses told local media they saw a truck driving into a department store window, and they have seen people on the ground.