Specially trained police from Gothenburg and Malmo backed up local officers in Stockholm, the BBC reported Saturday.
The disturbances began Sunday in Stockholm, but spread Friday to Orebro, where about 25 masked youths set fire to three cars and a school, and Sodertalje, where a vacant building was burned, police said.
One officer in Orebro was slightly injured by broken glass after youths threw rocks at police.
The riots are believed to have been sparked by the police shooting of an elderly man who had threatened them with a machete.
Gulan Avci, a member of the Swedish parliament of Kurdish origin, blamed the riots on a mix of criminality and youths from immigrant neighborhoods frustrated by high unemployment and poor schools.
She said Sweden's immigration policies had not been successful "but you can never accept violence as a way to show your disappointment with society."