COPENHAGEN, Denmark (AP) — Masked youths torched dozens of cars overnight in Sweden and threw rocks at police, prompting an angry response from the prime minister, who denounced an "extremely organized" night of vandalism.

About 80 cars were set ablaze overnight, chiefly in Sweden's second largest city, Goteborg, and nearby Trollhattan, an industrial city, and fires were also reported on a smaller scale in Malmo, Sweden's third largest city, police said Tuesday.

In Trollhattan, northeast of Goteborg, where at least six cars were burned, rocks were also thrown at police and roads were blocked. Goteborg is 400 kilometers (250 miles) southwest of Stockholm.

Police noted the fires started within a short period of time and believe "there is a connection between the blazes."

"As of now we have no motive whatsoever," police spokesman Christer Fuxborg told The Associated Press. "Our theory is that the fires have somehow been coordinated on social media like Snapchat but we do not know why."

Local newspaper Goteborg-Posten noted police in recent days have been active in pursuing drug dealers in Frolunda, a suburb of Goteborg where of some the fires took place.

"Honestly we do not know whether this has something to do with it," Fuxborg said.

Photos posted by Swedish tabloid Aftonbladet showed black-clad men torching cars on a parking lot near Goteborg.

Sweden's news agency TT said witnesses had seen "masked youngsters" running away.

Several youths that police met at the scene have been identified.

"We have spoken with them but we cannot conclude they started the fires. We also have spoken with their parents," Fuxborg said, adding police were in the early stages of the investigation.

Two people, aged 16 and 21 and living in Frolunda were detained for questioning, Fuxborg said. They later were formally arrested on suspicion of arson. More suspects likely could be detained.

Swedish Prime Minister Stefan Lofven lashed out at the perpetrators, asking them: "What the heck are you doing?"

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In an interview on Swedish radio, he said he was "really getting mad" and that "society must react in a tough manner." He said the fires seemed to be "extremely organized."

No injuries have been reported. However, the fires occupy police and rescue officials and frighten residents.

"You damage residential areas and ruin it for your neighbors," Lofven said.

"I am speechless. This so terrible, it's destructive and it's pure evil," Jonas Ransgaard, a member of the Goteborg City council, told local daily Goteborgs-Posten.