Cars and buildings were torched overnight in the medium-sized towns of Oerebro, Uppsala and Linkoeping, though tensions showed signs of easing in Stockholm's suburbs.
The unrest has 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.
Firefighters responded to about 30 or 40 incidents in the greater Stockholm area overnight, down from 70 the night before and 90 the night prior to that.
"The past night was the calmest we've seen so far," Stockholm police spokesman Kjell Lindgren said.
Police reinforcements were called in from Sweden's second and third biggest cities, Gothenburg and Malmoe – which have both experienced riots in the past decade – and volunteers patrolling the streets to restore calm had had a deterring effect and helped reduce the violence, Lindgren said.
"With the strong presence on the streets of the good forces, and the police reinforcements, I think we are well on our way towards calmer times in the coming days," he said.
Police arrested one person for attempted assault and about 20 others were briefly detained and then released for disturbing the peace, Lindgren said.
In the town of Oerebro, 160 kilometers (100 miles) west of Stockholm, police reported a fire at a school as well as several cars ablaze. A police officer was injured by a thrown stone and a police station was vandalised.
In Linkoeping, 235 kilometres southwest of the capital, a number of vehicles were incinerated, and a nursery and a primary school were both set on fire.
And in Uppsala, 70 kilometres north of Stockholm, a school and a car were set ablaze and a pharmacy was vandalised.
It remained unclear whether the cause of the unrest in the other towns was, as in Stockholm, immigrants' discontent, or merely copycat vandalism.
But Oerebro police spokesman said he believed it was the latter.
"I think some people are just taking advantage of the situation to commit these crimes as a result of what has happened in Stockholm and the attention that has received," he told TT.
About 200 right-wing extremists were reported to cruise around Stockholm suburbs in their cars late Friday, but intense police surveillance 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.
The troubles began in the suburb of Husby, where 80 percent of inhabitants are immigrants, believed to be 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 Swedish 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.
But 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 many of the rioters were 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 them struggle to learn the language and find employment, despite numerous government programmes.
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Sweden must stop its explotaion of immigrants.