Rakhmat Akilov, a 39-year-old Uzbek national believed to have sympathised with jihadist groups, is being held on suspicion of ramming a stolen lorry into pedestrians on a busy street in central Stockholm before crashing it into a department store.
Four people were killed and 15 others injured.
Akilov, a construction worker who had been refused permanent residency in Sweden, was arrested several hours after the attack in Marsta, a suburb north of Stockholm.
Local media, citing sources close to the inquiry, say he has confessed to the attack claiming he was ordered by the Islamic State group to target "infidels".
No group has yet claimed responsibility for the attack.
In a court document seen by AFP, Akilov's state-appointed lawyer Johan Eriksson said his client had asked him to "relieve himself of his mission", saying he "expressly wanted to be defended by a Sunni Muslim".
Akilov had argued that "only a lawyer of this faith could assert his interests in the best way", the statement said.
But Judge Malou Lindblom dismissed the request "for lack of sufficient reasons", in a written decision sent to AFP.
Akilov is due to appear in court at 10:00 am (0800 GMT) on Tuesday for a magistrate to decide whether to remand him in custody.
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