A "suspect device" was found in the driver's seat of the lorry used in the Stockholm attack, police have said.
At a press conference, detectives also revealed the 39-year-old suspect from Uzbekistan was known to security services but that the intelligence was "marginal".
The man is not thought to have spoken to detectives since he was arrested yesterday.
ITV News reporter Sally Lockwood said Swedish police are drawing "clear similarities" with the Westminster attack on 22 March.
The main suspect in the Stockholm lorry attack has admitted to carrying out a terror crime, his defence lawyer said.
The Ahlens department store in Stockholm apologised "for a bad decision" in promoting the offer.
The British man who was killed in the Stockholm lorry attack has been named as Chris Bevington.