Stockholm suburbs suffered a fifth straight night of unrest as rioters burned two schools and 15 cars, and tried to damage a police station, Swedish police say.

Police spokesman Kjell Lindgren said Friday that 13 people, aged between 18 and 25, had been detained after the disturbances.

Rioting on this scale is unprecedented for the Swedish capital and has raised questions about the country's attempts to integrate foreign-born residents, who now make up some 15 per cent of the population, the BBC reported.

Stockholm county police Chief Mats Loefving blamed local youths, both with and without criminal records.

"In the midst of all this there is a small group of professional criminals, who are taking advantage of the situation to commit crimes like this," the BBC cited him telling Swedish Radio.

A little quieter

The trouble began last week, sparked by perceptions of police brutality after officers shot and killed a knife-wielding man who had locked himself in his apartment.

Lindgren said Friday, however, that the overnight violence was less intense than previous nights, and that the participants seemed less aggressive.

"In terms of extent, it is a little less, a little quieter," Lindgren told Reuters.

Stockholm police got reinforcements Friday from the cites of Goteborg and Malmo in anticipation of more unrest, police spokeswoman Marie Hummer said. "We also need fresh officers as there is a busy weekend of events ahead, including a big horse race."

In one suburb, more than 80 per cent of the 12,000 or so inhabitants are from an immigrant background, and most are from Turkey, the Middle East and Somalia, the BBC reported.

Community activists have accused the police of using racist language. Police have tried to calm the situation by speaking with community leaders.

With files from The Associated Press and Reuters