Groups of youths in Sweden set fire to dozens of cars in the city of Gothenburg and surrounding towns on Monday, in what Prime Minister Stefan Lofven described as "extremely organized" attacks.

Police said up to 100 cars were burned or damaged in Gothenburg, Sweden's second-biggest city, as well as in Trollhattan, an industrial area with high unemployment, and Falkenberg. A number of cars were also burned overnight in Stockholm.

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Police did not say what might have motivated the "organized and prepared" attacks, only confirming that gangs of youths were involved without specifying numbers.

Witnesses told police the alleged offenders were dressed in dark clothing and hoodies.

'Society must react'

"It seems very organized, almost like a military operation," Lofven said in an interview on Swedish radio, but did not say who might have been behind the attacks.

"Society is always going to react strongly to this kind of thing," he said.

Lofven asked the perpetrators: "What the heck are you doing?" before going on to say that he was "really getting mad" and "society must react in a tough manner."

No injuries were reported and no one has been arrested, the Swedish Aftonbladet newspaper reported.

Violence fuels election campaigns

Public concern over gang-related and other violence has become a key issue in political campaigns ahead of Sweden's national election on September 9.

A surge in gang violence in Sweden saw more than 40 people shot dead last year.

The concerns could benefit the country's populist, anti-immigration Sweden Democrats in particular, giving them the potential to become the second-biggest party in the parliament.

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In addition to escalated violence, voters are worried about a struggling welfare system, declining school results and the effects of immigration, which saw around 160,000 people seek asylum in Sweden in 2015.

The police killing of a 69-year-old man who wielded a machete in a suburb of Stockholm, prompted accusations of police brutality in 2013 and saw rioting by hundreds of youths in the capital.

law/kms (AP, dpa, Reuters)