The man, who was arrested in the north of the city, has not been named, but Swedish media say he is from Uzbekistan, BBC reported.
Ihrman and police spokesman Lars Bystrom said only one person had been arrested in the case.
Hundreds of flower bouquets covered the steps leading down to the square next to where the truck ploughed into the Ahlens department store, with more piled up under boarded-up windows. Witnesses said that the driver was trying to hit people as he steered.
Earlier: The Swedish truck attack suspect was an asylum seeker who had his application rejected, Stockholm police said.
Mr Evenssen said police were holding five more people in connection with the deadly attack, after raids over the weekend, and that they had conducted about 500 interviews as part of the inquiry.More news: Bill O'Reilly gets axed by Fox
Speaking at a press conference on Saturday, the head of Sweden's National police, Dan Eliason, said security officials had found an object in the truck that "could be a bomb or an incendiary object".
"And he's been wanted by the police though he's not been in custody of the migration office", said National Strategic Commander Jonas Hysing.
The U.S. State Department also condemned the attack, adding, "Attacks like this are meant to sow the seeds of fear, but in fact they only strengthen our shared resolve to combat terrorism around the world".
Four people, including a British man, a Belgian woman, and two Swedes, were killed on Friday (local time) when a stolen delivery truck crashed into crowds in central Stockholm.
The assault took place in Drottninggatan, one of the city's main pedestrian areas, on Friday, killing four and injuring over a dozen people.More news: U.S. tells North Korea: We don't want a fight, don't start one
There was no evidence to suggest any further attacks.
"Firstly, to those of you who today are mourning a loss or are anxious about someone who is injured, I want you to know that Sweden is by your side", said Lofven.
"Today, all of Sweden is in mourning but we are going to get through this together", he said last Saturday.
There was no immediate claim of responsibility for Friday's attack.
On Sunday, thousands of people gathered in central Stockholm for a "Lovefest" vigil against terrorism, and laid flowers outside the Ahlens shop in tribute.More news: EPA to reconsider oil and gas industry emissions requirements
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