"They seemed to hate me very strongly," Sahlin said, explaining that she recognized the men after having participated in a demonstration earlier in the day.
The incident was witnessed by veteran Expressen reporter K-G Bergström.
"Then came three people who walked past us, but one of them stopped a few metres feet away, staring at Mona. She calmly turned around and asked if he wanted anything. And then came the 'traitor'," Bergström told the TT news agency.
There is a well-documented file of threats directed towards Sahlin, who became something of an object of hate among right-wing circles for her stance against racism and intolerance of immigrant communities in Sweden.
Since resigning as party leader in April 2011 and subsequently leaving the Riksdag Sahlin has however been without protection from Swedish security service Säpo and she was alone in Visby at the time.
Bergström said that Mona Sahlin had told him that this type of situation was a fact of life for her since leaving formal Swedish politics.
"She is incredibly vulnerable because of her courageous fight for tolerance towards immigrants. What would Säpo say if something happened to her?" K-G Bergström said.
Prime Minister Fredrik Reinfeldt reserved warm praise for his former political adversary in his speech on Wednesday. He gave Mona Sahlin credit for a consistent stance against racism, and the Sweden Democrats.