The festivities ended in violence as two people were stabbed within the span of half an hour. The incidents took place towards the end of the event, which drew up to 15,000 people to Stockholm's Kungsträdgården square, according to figures from the police.
The victims were brought to hospital, but were later discharged.
Police believe the disturbances may be connected to a group of between 200 and 300 younger men who had travelled to Stockholm from Uppsala. They were later escorted on the train back to Uppsala, where local police took over.
"During the evening, a number of young men were causing trouble and we saw two stabbings, which we have filed as attempted murders, but the victims' injuries were not serious," Lars Byström, a spokesperson for the Stockholm police, told Radio Sweden.
An on-duty police commander told Swedish Radio on Tuesday evening that the group consisted of unaccompanied minor refugees. Asked whether that is correct, Byström said "it is not completely unlikely".
Apart from the stabbings, one case of sexual harassment was also reported at Tuesday's Festival of Fire. Police have identified the suspect, who, if found guilty, is likely to be sentenced to pay a fine.
The Festival of Fire has been celebrated in Kungsträdgården since 2000. According to the organisers, it is the biggest celebration of its kind outside Iran. For the first time, Swedish Television chose to broadcast live from the festival this year and it was generally a very pleasant event, said Byström.
"The trouble came from these younger men who turned up...No shadow should fall on the festival itself," Byström insisted.
No one has been arrested for the stabbings, but there were apparently several witnesses to one of the incidents.