Ronja Ferm was one of many car enthusiasts enjoying a cruising meet-up on Sveavägen in central Stockholm last weekend.
They were playing music and dancing along in their car, so when a uniformed police officer walked up to the vehicle, she expected a telling-off.
Instead, she was asked to dance.
Photographer Petra Griner Lewén managed to snap a picture of the two strangers' improvised dance and it has now gone viral in Sweden. Keep reading to find out how the police officer got the idea in the first place.
Anton Larsson, normally a police officer on the Rinkeby beat in northern Stockholm, had been called in to help monitor traffic at Saturday's cruise and told The Local he got caught up in the joyous mood at the event.
"It was a spur of the moment idea. The atmosphere, the music, and then the opportunity arose and I took it. Dancing is so nice and spreads such joy, both to yourself and others," he said.
Griner Lewén told The Local she is planning to frame the picture so that Larsson can hang it at the police station, where it has also drawn attention from his colleagues.
The policeman himself said he had not anticipated this many reactions.
"It's been bigger than I ever could have imagined, very positive reactions both from the public and from colleagues. The uniform is not that unusual to me, but I understand that other people think it is extra fun when you act a bit more outgoing," he told The Local.
Larsson told the Aftonbladet tabloid, which first wrote about the image, that he had had to briefly halt the other cars for the dance.
His dance partner Ferm said she had been surprised when Larsson approached her.
"I wondered what it was he wanted when he popped his head through the window and gave me his hand. I thought he wanted me to do an alcohol test or tell us to calm down, because there was quite a lot of music. I certainly never thought he would ask me to dance," she told The Local.
"I am over the moon about how much the picture has spread and that it has made so many other people happy."
This is not the first time an unusual picture of a Swedish police officer goes viral. Last year off-duty Stockholm cop Mikaela Kellner hit international headlines after a friend snapped a picture of her arresting a pickpocket by wrestling him to the ground while wearing a bikini.
Earlier this summer, in Berlin, a video of a dancing policewoman became a hit on social media after she danced salsa with a visitor at the city's Carnival of Cultures.