The protests in Stockholm were triggered by the fatal police shooting of a 69-year-old resident last week.
Cars were set on fire in several suburbs overnight, prompting some countries to issue travel advisories.
Stockholm police have called for reinforcements from Gothenburg and Malmoe, the country's second- and third-largest cities.
The protests have rocked Stockholm’s immigrant-dominated neighborhoods, sparking a debate over the country’s asylum policies.
Immigrants now make up about 15 percent of the population in Sweden.
The deadly shooting has sparked anger among youths who claim to have suffered from police abrutality and racism.
Right-wing extremists have also become active, raising fears of fresh violence. About 200 extremists were reported to cruise around Stockholm suburbs in their cars late Friday.
Police are beginning to make arrests and have rounded up more than two dozen people so far.
The riots have mainly rocked Stockholm’s suburb of Husby, where 80 percent of inhabitants are immigrants.
Residents told Swedish Radio that racism was rampant where they lived, and that protests were their only way of being noticed.
Stockholm police say the rioters are local youths with and without criminal records.
Sweden has in recent decades become one of Europe's top destinations for immigrants, but many of those who have arrived struggle to learn the language and find employment.