At the halfway point in the race, Ethiopia's Bazu Worku, was in the lead, with the first Swedish national Nuray Yassin far behind in 13th place.
“It is not expected to go over 30 degrees, but it may feel like over 30 degrees in areas with a lot of asphalt,” the organisers wrote in a message to the 15,700 runners posted on the marathon’s website. “Make sure that you keep yourself cool properly during the race.”
Marathon press chief Lorenzo Nesi told Expressen that competitors should fully hydrate themselves on Saturday morning, both with water and with sports drinks to ensure they have sufficient salt and other minerals in their bodies.
“Set off at a leisurely pace and don’t think that this is the sort of day where you can beat your personal record,” he advised runners.
Some 6,300 of those who had applied for the race didn't show up, which Nesi told Swedish broadcaster SVT was an “unexpectedly small” number given the heat.
This April's London Marathon was the hottest ever, with dozens of competitors collapsing from heat exhaustion after running in 24.1C.
But Nesi said the Stockholm competition had taken place in similar hot sunny weather in 2007.
The marathon, which started at 12am, has drawn in 20,000 runners from 103 countries. It is the 40th time the competition has been held.
“I think it will be a nice day. We have more women this year than we have had in previous events,” Nesi said.
The route starts and finishes at Stockholm’s 1912 Olympic Stadium, and goes through the upmarket districts of Östermalm and Norrmalm, and across the island of Kungsholmen, before going over to and then around the island of Södermalm, back through the historic Gamla Stan, and around Djurgården.
But Saturday's heat in Stockholm is nothing compared to the Marathon des Sables, the 251km ultramarathon dubbed 'the toughest race on earth', which sees runners brave temperatures of 48C over six days.