"Another public gathering took place close to the festival site, during which a violent riot broke out," police said, adding in a statement they had detained "around a hundred people."
Police said they remained at the scene in a suburb northwest of Stockholm and were "continuing their efforts to disrupt criminal acts and restore order." They said they had also opened an investigation into violent rioting and arson as well as obstructing the work of police and rescue services.
Around 1,000 anti-government demonstrators authorized to hold a protest nearby broke through a police barrier as they stormed the festival, the tabloid Expressen reported.
They tore down festival tents and used tent spikes as weapons against police as well as throwing stones at officers, the paper said.
By 1500 GMT seven people had been taken to hospital, regional healthcare authority Region Stockholm said in a separate statement.
Four of them had "serious injuries," with the other three having sustained "minor injuries," according to the authority, which added it had multiple units at the scene.
"It is a complicated and extensive operation. There are a lot of people in motion at the site and the total number of injuries is still unclear," Patrik Soderberg, chief physician at Region Stockholm, was quoted saying.
Footage from the scene showed cars and at least one tent on fire, sending large clouds of black smoke into the air.
Police shut down a section of the nearby E18 motorway in both directions as people fleeing the scene blocked the road.
The Eritrea-Scandinavia festival, which has been held for many years, features seminars, debates and lectures, as well as music, a bazaar and a fairground.
It is scheduled to run from Thursday to Sunday.
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