Swedish police arrested three people on Monday on suspicion of preparing "an act of terror" and took in several more for questioning after raids in northern Sweden and the Stockholm area, the security police said.
It added there were no indications that any attack was planned for the next few days.
"Preparations for the suspected crime have been ongoing for some time," the security police said.
The terrorist threat level remained unchanged at three on a scale of five, which is "an increased threat."
Security police declined to give further comment.
But the domestic intelligence agency SAPO said the suspects had been on the agency's radar for a while.
The agency said several other people have been brought in for questioning.
In April last year an asylum seeker killed five people and injured 14 others when he mowed down pedestrians with a truck on a busy Stockholm street. He is on trial and has admitted to driving the truck.
