STOCKHOLM (Reuters) - Youths torched and vandalized scores of cars in the Swedish city of Gothenburg and surrounding towns and Prime Minister Stefan Lofven said on Tuesday the disturbances looked organized “almost like a military operation”.

Burned cars are pictured at Frolunda Square in Gothenburg, Sweden, August 14, 2018. Adam Ihse/TT News Agency/via REUTERS

It was unclear what triggered Monday night’s unrest but Sweden has seen a rise in violence in areas with high unemployment and other social problems and the incident echoed rioting in the capital Stockholm in 2013.

Mounting public concern about gang-related and other violence has become a central issue in campaigning ahead of a national election on Sept. 9.

Police said that up to 100 cars were either set fire to or vandalized in Gothenburg, Sweden’s second-biggest city located in the western part of the country, and in nearby towns such as Falkenberg and Trollhattan, an industrial area with high unemployment. There were no reports of injuries.

Two men, 16 and 21 years old, were arrested on Tuesday and police said it expected to make more arrests during the day.

Police said gangs were involved but gave no details.

Swedish towns have seen sporadic violence on a smaller scale in mid-August in recent years at the end of the school summer break.

“We believe that up to 100 cars have been burned or damaged,” police spokeswoman Ulla Brehm said.

“They were organized and prepared.”

A handful of cars were also burned overnight in Stockholm.

“It seems very organized, almost like a military operation,” Lofven said in an interview on Swedish radio. “Society is always going to react strongly to this kind of thing.” Lofven visited the region on Tuesday and met police.

The violence echoed rioting by hundreds of youths in Stockholm and some of its poorer suburbs in 2013, sparked by the police killing of a 69-year-old man wielding a machete in a suburb that prompted accusations of police brutality.

Law and order is a key issue ahead of the election after a spike in gang violence saw more than 40 people shot dead last year.

Despite economic growth that has for years been the envy of most of Europe, Lofven’s Social Democrats, the bigger party in the current minority coalition, are on track for their worst ever election result, according to opinion polls.

Voters also worry about the welfare system, falling school results and the effects of a surge in immigration that saw around 160,000 people seek asylum in Sweden in 2015.

The concerns have benefited the populist, anti-immigration Sweden Democrats who look poised to become one of the three biggest parties in parliament after the vote.

Sweden's general election: tmsnrt.rs/2LmSZFD