Despite its off-putting name, Ruttopuisto (Plague Park) is a pleasant patch of greenery in the heart of Helsinki. At 11am on a sunny Saturday in May, its appeal was enhanced by an outcrop of tasty offerings. At one stall, Johannes Dunbel sold bagels made in the authentic way ("yes, they're boiled") and filled with either prosciutto and tomato or pesto and hummus. Since I had the foresight to skip breakfast, I was able to try both. Excellent. "I only had three hours' sleep last night," said Johannes. "We made 100 bagels and have already sold 70 in 45 minutes."
"I've never been so stressed," admitted his female partner.
The stall was part of Helsinki's Restaurant Day, a four-times-a-year event for people who want to try their hand at catering. (You can get a taste for the concept next weekend.) Johannes, who normally spends his days studying for a PhD in tumour immunology, provided a cheering explanation for his deviation into bagels: "You need to bring happiness to people."
Other pop-ups in the park were run by housewife Marja Aspelund, who offered Mediterranean muffins with sour-sweet additions of lemon, olives and cheese. Then, there was restaurant worker Oskari Sandstrom, whose freshly fried blinis, topped with smoked salmon and cream cheese, were so good that I could happily have spent the rest of the day snacking there.
But there was plenty more to sample in Helsinki's outdoor banquet. A dozen stalls punctuated Iso Roobertinkatu, one of the city's main shopping streets. Artist Tarja Saikkonen was selling heart-shaped waffles smothered with strawberry jam. A pop-up Mexican restaurant called El Puesto drew a 40-strong queue for its tacos (chicken and tomato or vegetable and mushroom).
Outside her father's snack bar, Balzac, 10-year-old Kreeta Koskijolei was selling madeleines from a stall named La Fille de Balzac. "They're the bee's knees," I told her. After a brief elucidation, she beamed. "I'm so pleased."
Throughout this elegant city of half a million, the pop-ups produced cheerful chat and contented chewing. This was the aim when the first Restaurant Day was held in May 2011, when 40 pop-ups appeared. But there was a more subversive intention, as one of the three founders explained. "We have many regulations that prevent the street food enjoyed in other countries," said Timo Santala. "We thought: what if just for one day these rules didn't exist and we could do all these things we dream of. Maybe 25,000 people offer food on Restaurant Day ... in a population of half a million, it's a tremendous number."
The authorities usually turn a blind eye on Restaurant Day, but an indication of the deep-rooted Finnish desire for order was evident at a solitary stall on a long street lined with apartments. At first, I thought the police car had stopped for a snack but it turned out that the cops had been called by a local resident fearing the anarchy that might result from the sale of American cakes (carrot, Boston cream, peanut and banana) made by students Mirkka Ollikainen and Annamari Jukkolalii. On finding that only waistlines were threatened, the law moved on.
A year on from the first Restaurant Day, 680 temporary restaurants took part. About half were in Helsinki; the rest were scattered around Finland with a sprinkling as far afield as Germany, Switzerland, the US and Canada. Hoping that the concept will spread, Timo and his colleagues invited potential organisers from cities including London, New York and Tokyo. However, Restaurant Day seems particularly suited to the city where it was invented. Helsinki is the ideal size and there is a deep desire to enjoy the customarily prohibited street food.
The variety on offer encourages a snack-punctuated tour of this city where Russian classicism rubs shoulders with Twenties Art Deco and cutting-edge design. (Helsinki is also this year's World Design Capital.) In her lingerie boutique on Punavuorenkatu street, Tyra Therman served a somewhat surreal afternoon tea. Amid racks of silken scanties, I sipped Earl Grey with the full accompaniment of scones, strawberry jam and cream.
In Bergmansgatan park, Matthew Rusting-Morey from Los Angeles and his girlfriend Maria Peltonen had a stall called Mekong Munchies selling salmon and vegetable summer rolls (like spring rolls but cold). Irresistibly light and delicious, just a couple remained. "We have sold 100 in 95 minutes," said Matthew. "I wish there was something like this in LA." In a reclaimed industrial area at the seaside, a serious pop-up called B Smokery served generous cuts of smoked pork from half a dozen big American smokers. "We marinaded 140kg, enough for 600 portions," said Mark Luoff. Were they professional cooks? "No, we work in a film company. There's the cameraman. He's a screenwriter, and I'm the producer."
Mark admitted that the outfit was due to open a restaurant downstairs from its new production suite in Helsinki's Meatpacking District. "I've never had so much fun in my life. It's really good to do this and bring lots of people together."
My final pop-up of Restaurant Day was on a small, ancient wooden jetty in Helsinki harbour, where there was a cheese and wine stall that added interest with a temporary sauna in a well-worn tent. As the sunset rippled on the gently lapping water, a stream of young people tripped into the steamy enclave as naked as Adam and Eve. An interesting and likeable people, the Finns may claim to be confined by rules but in many ways they are freer than we are.
Helsinki airport is served by Finnair (0870 241 4411; finnair.com) from Manchester and Heathrow; British Airways (0844 493 0787; ba.com) and Blue1 (00 358 20 585 6240; blue1.com) fly from Heathrow; and Norwegian (00 47 21 49 00 15; norwegian.com) flies from Gatwick.
More information The next Restaurant Day is on 19 August, with another in November: restaurantday.org; visithelsinki.fi