Nearly a week of unrest, which spread briefly Friday night to the medium-sized city of Örebro 160 kilometres west of Stockholm, have put Sweden's reputation as an oasis of peace and harmony at risk.
The unrest has also sparked a debate among Swedes over the integration of immigrants, many of whom arrived under the country's generous asylum policies,
and who now make up about 15 percent of the population.
An AFP photographer witnessed a car engulfed in flames before firefighters arrived in the Stockholm district of Tensta. Cars were incinerated in three other areas of the capital as well, according to the Swedish news agency TT.
"I've never before taken part in anything that lasted so long and was spread over such a wide area," police spokesman Lars Byström told TT.
Another police official said earlier that Stockholm police were about to receive reinforcements from Gothenburg and Malmö, the country's second- and third-largest cities, but declined to disclose how many would arrive.
In the city of Örebro, police reported a fire at a school as well as several cars ablaze, but quiet had returned around midnight. The unrest in Stockholm had "rubbed off", police told TT.
About 200 right-wing extremists were reported to cruise around Stockholm suburbs in their cars late Friday, but intense police surveillance apparently prevented any kind of serious violence.
The nightly riots have prompted Britain's Foreign Office and the US embassy in Stockholm to issue warnings to their nationals, urging them to avoid the affected suburbs.
Firefighters were dispatched to 70 fires the night between Thursday and Friday, extinguishing torched cars, dumpsters and buildings, including three schools and a police station, the fire department wrote on Twitter. This was down from 90 blazes the night before.
Parents and volunteer organisations who have patrolled the streets in recent nights have helped decrease the intensity of the unrest, police have said.
Police, who have so far concentrated on putting out fires, are beginning to round up people suspected of criminal acts, so far arresting at least 29.
"Even if we don't intervene, we regularly make video recordings and get information from the public. That way we can get people a couple of days later," police spokesman Byström told TT.
The troubles had begun in the suburb of Husby, where 80 percent of inhabitants are immigrants, triggered by the fatal police shooting of a 69-year-old Husby resident last week after the man wielded a machete in public.
Local activists said the shooting sparked anger among youths who claim to have suffered from police brutality and racism.
One of the rioters in Husby told Sveriges Radio that racism was rampant where he lived, and that violence was his only way of being noticed.
"We burned cars, threw rocks at police, at police cars. But it's good, because now people know what Husby is... This is the only way to be heard," said the rioter, identified only by the pseudonym Kim.
Stockholm county police chief Mats Löfving said Friday the rioters were local youths with and without criminal records.
In addition, "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," he told Sveriges Radio.
A 25-year-old who grew up in Husby said he didn't think the riots had anything to do with the shooting.
"I'm not saying there are no problems... but people are glorifying this a little bit," said the man, who declined to be named, adding that the rioters were often aged 12 to 17.
"I can imagine they get a big kick out of seeing themselves on TV," he said.
Due to its liberal immigration policy, Sweden has in recent decades become one of Europe's top destinations for immigrants, both in absolute numbers and relative to its size.
But many of those who have arrived struggle to learn the language and find employment, despite numerous government programmes.
Official data show unemployment was 8.8 percent in Husby in 2012, compared to 3.3 percent in Stockholm as a whole.
Eric Zemmour, a right-wing French commentator known for his controversial views, meanwhile told RTL radio the riots showed that the Swedish "kingdom of social democracy and of political correctness" was little different from countries like Britain and France.