In Gothenburg eight cars were set alight while three vehicles were destroyed in Malmo on Wednesday night.
Despite being called to the scene shortly after the blazes were set, police said they have no leads or suspected in the case.
Initially, emergency services were told four cars had been set alight but before they could arrive, another two vehicles had caught fire.
Police spokesman Thomas Gorner told Expressen: “A total of six cars were damaged in the fire.
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Sweden is facing a growing epidemic as arsonist thugs set cars ablaze
“It appears that someone started a fire and it spread. We have tried to gather information from witnesses, but we have no suspects at this time.”
In addition to the six cars destroyed in the Frölunda parking lot in the south-west of Gothenburg, another two burned out across the city in the suburb of Brunnsbo.
Dan Lindqvist, a spokesman for the emergency services, said a seventh car has also been affected by the inferno.
He said: “Six cars were burning when we arrived. Car fires rapidly spread, these are incredible temperatures.
Car fires rapidly spread, these are incredible temperatures
“A seventh car was fire-affected by smoke and heat radiation.”
Vandals also struck in Malmo, Sweden’s third-largest city, during the night as three vehicles burned out in the early hours of the morning.
Malmo has been plagued by increasing levels of violence amid growing tensions as police have tried to crack down on criminal activity.
During three months in 2016, over 70 cars were set on fire in an act of retaliation against police.
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At the time, Officer Erik Jansåker said: “This, in my opinion, is criminal people who are [lashing out] because of our efforts to [crack down] on the serious organised crime.”
Despite great efforts to halt the arsonist attacks in Malmo has been described as a “lawless city” and a “battlefield” as the surge in crime continues.
Witness Freddy Mardell described Malmo as “a lost city” as he insisted Swedish teenagers had to cling together in large groups not to be robbed.
A number of Swedish cities are facing a surge in car fires
Admitting it is not safe to enter the Malmo square on NYE, police spokesman Nils Norling said: “It’s known that you can’t guarantee your own safety if you’re there on New Year’s Eve. There’s a risk you could be hit with a rocket.
In 2016 Swedish police issued a report where it detailed incidents from more than 55 areas which it branded as “no-go zones”.
It detailed brutal attacks on police, sexual assaults, children carrying weapons and general turmoil sweeping across the country.
Over the weekend, police in Gothenburg admitted the liberal country’s largest shopping centre had turned into a “no-go zone” as migrant youth gangs terrorising locals and business owners, as well as threatening officers and harassing girls.