In the Swedish city of Gothenburg on Monday night, plumes of smoke rose above the shells of burned cars as fire engines raced to put out a string of blazes that were captured on camera.

More than 100 cars were "burned or damaged" in and around the nation's second-largest city, police spokeswoman Ulla Brehm said, after youths reportedly moved through the area Monday evening torching passenger vehicles.

"They were organised and prepared," Brehm said, according to Reuters.

ADAM IHSE/AP Masked youths torched dozens of cars overnight in Sweden and threw rocks at police, prompting an angry response from the prime minister.

No injuries were reported, but photos show that many cars were destroyed.

On Tuesday, police said they had arrested a 16-year-old and 21-year-old in relation to Monday's attacks, and planned to detain more suspects during the day.

Prime Minister Stefan Lofven said on Swedish radio that the fires appeared to be "very organised, almost like a military operation."

"What the heck are you doing?" Lofven asked in the interview, as though addressing those who set the fires, adding that he is "really getting mad."

Gangs were likely involved in the string of arson attacks, police said, and they come amid mounting concerns in Sweden about gang-related violence.

More than 40 people were shot and killed in the Nordic country last year, and Lofven said in January that he was not ruling out a military response to gang activity.

"It would not be my first option to bring in the military, but I am prepared to do whatever is necessary to make sure that serious, organised crime is stamped out," he told the TT news agency at the time.

In response to Monday's incidents, Lofven said that "society must react in a tough manner."

Populist politicians have alleged that increased crime in Sweden is linked to migration. The Sweden Democrats, a far-right party with links to neo-Nazism, have suggested freezing out asylum seekers and reducing the country's immigration budget.

But when it comes to intentionally setting fire to cars, said Manne Gerell, a lecturer in criminology at Malmo University, such incidents have increased "big-time" over the past couple decades - but not in the past two or so years.

Across Sweden, around 1500 cars are burned each year, he said. The reasons can range from insurance fraud or hiding other crimes to social unrest and "random youth vandalism."

In 2013, days of riots broke out in the capital, Stockholm, after police were accused of brutality for shooting and killing a 69-year-old man holding a machete. For days, rioters burned cars and buildings, including at least one school. They also threw rocks at police.

For now, what seems to be missing in this week's case is a clear motive.

"Usually when we have widespread incidents of torched cars, it's usually related to social unrest," Gerell said. This time, "it's nothing concrete."

In early 2017, riots broke out in a largely immigrant neighbourhood near Stockholm. As The Washington Post's Max Bearak reported then, the riots occurred "just two days after US President [Donald] Trump provoked widespread consternation by seeming to imply, incorrectly, that immigrants had perpetrated a recent spate of violence in Sweden."

Sweden is far from the only place where youth have set cars on fire. On recent New Year's Eves in France, young people have torched hundreds of cars. During recent end-of-year celebrations, around 1000 cars were set on fire a number of years in a row.