Asplund's defiant act only lasted for a couple of seconds -- just enough for Swedish photographer David Lagerlöf to capture the image.
"When I did it, I was angry. I wasn't scared," Asplund told CNN. "Now when I think about it, I understand it could have been worse. Now I see that."
The march in Borlänge was organized by the Nordic Resistance Movement (NRM), a political party that opposes immigration to Sweden by non-whites. The marchers were parading down a street when Asplund stepped out in front of them and raised her fist briefly before police pushed her away.
"She was in real danger," said Lagerlöf. "She had nothing to stand up to those guys. The Nazis that she confronted, there were many of them, they were bigger, they are used to violence, and what she did was she went up to them, stood in front of them and stared at the leaders, stared them in the eyes, and it should have ended really bad. "
A portrait of courage
The episode happened so quickly that Lagerlöf didn't realize how good the photo was until he got home and started checking over his work. It soon became evident "that this was the one," he told CNN.
He posted it early on Tuesday to Twitter, where it has since been shared more than 3,100 times -- including by a rather famous British author.
"Look what this woman did. Tess Asplund, you are magnificent," tweeted J.K. Rowling, creator of the Harry Potter series. Her tweet was shared more than 8,000 times.
After the incident, Asplund went home and didn't give the march another thought, she says. But by Tuesday morning, her photo had gone around the world and her phone was ringing off the hook.
"I'm still in shock, because I don't understand it," she said.
Lagerlöf also didn't expect such a fuss when he took the photo and still doesn't know to what extent it has been distributed worldwide. But "when it reaches J.K. Rowling -- I don't know if it can get any bigger than that," he said.
Does Lagerlöf think this image can reach the heights of other celebrated protest photographs?
"I know that there's a lot of speculation about how iconic it might be, but I would take that with a pinch of salt. There's a lot of pictures being produced right now, by a lot of brilliant photographers, and there's a flow of really good photography," he said. "So we'll just have to wait and see."
'It was just impulse'
Lagerlöf saw other people protesting against the march, including a group that dressed like clowns to divert attention away from the marchers.
"I've been to several other demonstrations and there has been lots more violence, with more clashes between anti-fascist groups and neo-Nazi groups," he said. "It was kind of calm this time."
Then Asplund showed up, breaking the ranks of the marching men and just standing there.
"They continued walking towards her and she backed a bit when they came too close," he said.
So what made this slight-framed woman step in the way of the march?
"It was just impulse, to go in their middle. I remember standing there and one of the guys staring at me," Asplund told CNN. "When you have Nazis marching in the street for May 1
, it's important to show that that's not okay. People in other countries can't understand how come Nazis are marching in Sweden."
Asplund says she's pleased that the photo has sparked a reaction.
"I'm happy because I've been fighting against racism for 26 years, and my act can make other people fight as well," she said.
"I hope people can see this still happens in our country right now," she added. "I hope that it can change things. I have always taken part in these demonstrations -- this is my Nelson Mandela cause. He's my mentor. And whatever I do about freedom and justice, it's for him, in his honor."
Is she scared at all about a backlash?
"Nothing has happened to me personally, not yet, but I know that they're very angry and I know that on their website, they're writing things. But I don't want to read that. Of course I understand them, they're Nazis, they're angry."
'A polarized society'
Swedish media have been quick to find a precedent
for Asplund: a woman immortalized on camera in 1985 while hitting a man over the head with her handbag during a Nordic Reich party march in the Swedish town of Växjö.
Lagerlöf thinks his picture carries a strong message.
"People react in an encouraging way when someone actually stands up to these racists, like she did. That's one of the reasons why I think it has spread," he said. "People are impressed by what she has done, and I guess it gives hope in a certain way."
So why is this happening in Sweden? Recent woes over the European refugee crisis
have triggered a rise in far-right extremism, propelling the right-wing Swedish Democrat party to the country's parliament.
"We live in a polarized society ... and lots of people feel like racist movements are gaining ground right now," Lagerlöf said.
He describes the NRM as "a Nazi group that is very aggressive. It's a radical national-socialist movement, a Nazi movement that is very hardcore. They see themselves as if they are the saviors of the nation. If you stand in their way, you might end up in some violence."