Gothenburg police and fire services were alerted to the first blaze after 9pm, after which several more calls came in from the city as well as Trollhättan, Lysekil and Falkenberg some 100 kilometres away.
“We have been to around 20 places in Gothenburg. It's mainly vehicles that have burned – cars, some truck, caravans – but also some buried waste disposal site,” Johan Eklund, emergency control room officer in the greater Gothenburg area, told Swedish news agency TT shortly after midnight.
Swedish media reported that groups of up to ten youths had been seen throwing stones and lighting cars on fire in Gothenburg districts Gårdsten, Hjällbo and Frölunda, among other locations.
Videos sent to The Local showed masked people dressed in black torching cars.
One witness, who asked to be named only as Robert, told The Local he had seen a group of half a dozen people torch cars in the car park outside his home in Frölunda. His wife called the emergency services.
“We were very surprised. This area is usually very calm. Nothing like this has ever happened before. Six months ago it happened on the other side of the square. We've been living here for eight years,” he said.
“They acted like a team, absolutely. It was organized. They knew exactly what they were going to do.”
In Trollhättan, 70 kilometres north of Gothenburg, a larger group of people was reportedly involved, wrote TT. Six cars burned in the town and youths were reported throwing stones at police.
“We know from experience that these kinds of fires more often happen the week before schools start than other weeks,” police spokesperson Ulla Brehm told TT.
Gothenburg police said on Tuesday morning they believed the attacks had been coordinated on social media, but added that they were still investigating and were not yet able to confirm that that was the case.
In total around 80 cars were set on fire across the region, said Gothenburg police.
Police said they had managed to identify several people believed to have been present at the scene of some of the fires. Just after noon on Tuesday they said two people, residents of Frölunda and aged 16 and 21, were being held in connection with the fires. They were later formally arrested on suspicion of aggravated arson.
Car burnings were also reported to a smaller extent in Malmö and Helsingborg. Southern police spokesperson Fredrik Bratt told TT it was “not completely out of the ordinary” and added he did not believe they were linked to the Gothenburg fires, but said they were monitoring the situation.
Around a dozen cars also burned in Stockholm, Uppsala and Åkersberga on Monday night, wrote the Expressen tabloid. There were no other reports of riots or violence in those areas, it added.
Reporting by Emma Löfgren and Viktoriia Zhuhan.