A lorry has driven into a store in central Stockholm, killing at least three people, Swedish media say.
Shots have also reportedly been fired.
Swedish police said a number of people were injured, and they are treating it as terrorism.
The incident occurred on Drottninggatan (Queen Street), one of the city's major pedestrian streets, just before 15:00 local time (13:00 GMT).
thanks to brave Swedes or what ever, who followed the driver of the truck as he tried to escape and then got in contact with the police.
Stockholm police said the suspected attacker, arrested later on Friday, had previously been sought by authorities for deportation.
He had sought residency in Sweden in 2014, but his application was rejected last year. He had expressed support for extremist organisations including the Islamic State group, police said.
While he was known to security services, he was seen only as a "marginal character", Sweden's National Police Commissioner Dan Eliasson added.
Meanwhile, a second suspect has been placed under formal arrest.
Reuters news agency said it was on "a lower degree of suspicion" than the first suspect.
Police have interviewed more than 500 people over the incident, Sweden's TT news agency reports.
Thousands of people have gathered in a square in central Stockholm on Sunday for a vigil against terrorism.
A British man killed in the Stockholm lorry attack has been named as 41-year-old Chris Bevington.
Two Swedes and one Belgian also died in the attack on Friday, when a hijacked lorry was driven into a store.
His family said they were devastated by the "untimely and tragic death" of the "wonderful husband, son, father, brother and close friend to many".