A truck ran into a crowd on a shopping street and crashed into a department store in central Stockholm on Friday, killing four people and wounding 15 in what the prime minister said appeared to be a terrorist attack.
Swedish prosecutors said a man detained in connection with a truck attack in the capital had been arrested on the strongest degree of suspicion of committing a terror crime.
"One person has been arrested on suspicion of terrorist crimes through murder," Karin Rosander, press spokesperson at the prosecutors office said.
Swedish public broadcaster SVT reported police had arrested a second man and that he had a connection to the previously arrested person, citing police sources. The police declined to comment on whether it had arrested any additional suspects.
"Our message will always be clear: you will not defeat us, you will not govern our lives, you will never, ever win," Prime Minister Stefan Lofven, who had earlier described the assault as a terrorist attack, told a news conference.
Police said security at Swedish borders had been heightened. They did not rule out the possibility other attackers were involved.
The area of the attack in central Stockholm was evacuated, including the main rail station, and remained cordoned off late on Friday. All subway traffic was halted on police orders and government offices were closed.
Bloody tyre tracks showed the path of the truck, which was stolen by a masked hijacker while making a beer delivery to a tapas bar further up Drottninggatan, according to Spendrups Brewery spokesman Marten Lyth.
Police said four people had died and 15 were injured. National news agency TT said those hurt included the delivery driver, who had tried to stop the hijack.