"He admits that he has stabbed the plaintiff twice," said defence lawyer Björn Törnell. "He did not intend to kill, only to cause minor injury."
The 35-year-old reportedly belongs to the organization the Revolutionary Front, which is considered left-wing extremist. He has several previous convictions behind him.
In court on Tuesday, he faced charges that he was among a group of black-clad demonstrators who went on the counter-attack when the neo-Nazi Swedish Resistance Movement threw firecrackers and bottles at an until-then peaceful neighbourhood demonstration against racism.
The court was shown videos from the altercation, with large throngs of people pushing up against each other in apparent attempts to repel the opposing group. At times, the scene descends into violence as individuals get into fights. The words "Adolf Hitler" and "Sieg Heil" are clearly audible above the din, the TT news agency reported from court.
The prosecutor has argued that the 35-year-old suspect showed up in Kärrtorp armed with a morakniv, a traditional Swedish knife used for wood carving and odd jobs.
"It was a heated atmosphere on our side also, of course, but that's because they (the neo-Nazis) arrived," the 35-year-old said in court. "It was calm before that."
The Revolutionary Front has printed FreeJoel T-shirts, amassed pictures showing support for the suspect, as well as organizing protests. Demonstrators held banners stating that "anti-fascism is always self-defence".
In court, the suspect explained that the neo-Nazis were the ones to go on the attack, which a recent verdict against seven of the Swedish Resistance Movement (SMR) members also stated, and that some people in the crowd had decided to fight back.
"I don't know what choice we had. Either to stand there and get beaten up, or defend ourselves," he said.
The suspect argued that the insufficient police presence - admitted to by the police themselves after the event - had left them little choice.
The plaintiff, also present in court on Tuesday, said that he did not at first notice that he had been stabbed, only later feeling the blood on his back.
A police officer also testified in court, and said the altercation was among the worst the officer had seen despite working at several football derbies in the past. The officer said the police on the ground received orders to keep the two groups apart.
The violence quickly became considerable, however.
"Unfortunately, SMR didn't back off..... they should flee to not be beaten to death, but they don't," the police officer recalled.