When the owner of the Mercedes locked their car on Wormser Strasse, Berlin on Thursday night, they probably knew it was risky. More than 370 cars have been set alight in the city this year, with the flashiest models being the chief victims.

But with police in Berlin promising to put 500 officers and a helicopter in pursuit of the arsonists, the Mercedes' owner may have thought they could sleep easy. Especially since this was the chi-chi district of Schöneberg, just a few minutes walk from the KaDeWe department store, Berlin's answer to Harrods.

But no. In the early hours of Friday morning the elusive firebugs struck again and the Mercedes became the 371st car to be torched in Berlin this year. A few minutes later an Audi in nearby Keithstrasse became the 372nd.

A spokeswoman for the police said that of the 372 cars torched in 2011, 155 were being treated as "politically motivated" crimes.

"That can be for a number of reasons," she said. "Sometimes it's because the cars are particularly valuable, sometimes it's the area they were parked in, for example districts where we know there are leftwing groups who are against gentrification." She specified the borough of Friedrichshain-Kreuzberg, which is historically a stronghold of the city's angry left. So far only 13 suspects have been identified.

Burning cars has long been a popular nocturnal pastime in Berlin, but the number of attacks this year has become a headache for the city's administration. Until now the record year for car burnings was 2009, when 401 were set alight. Unless police quickly get a grip on the problem that record is likely to be smashed by the autumn. In a two-week period earlier this month 90 cars were targeted.

It has become the hot campaigning topic for the city and state elections taking place in September. Even the chancellor has become involved, saying she is watching developments "with great concern". One politician from Angela Merkel's Christian Democratic Union party has even floated the idea of Berliners forming a Bürgerwehr, a sort of unarmed citizens' militia. Burkard Dregger told voters that if the party triumphs in September he will kit out 1,000 volunteers with truncheons and handcuffs.

The idea was immediately rubbished by Germany's police union. "Bounty hunters belong in westerns," representative Klaus Eisenreich told the newspaper Die Welt. "Vigilante justice is a real risk. Solving the crime of car burning is a job for the police."

Berlin is not the only city in Germany facing the problem. On Wednesday morning four cars were set alight in the usually more obedient city of Düsseldorf, and Hamburg has also seen a number of attacks.