"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 authorised to hold a protest nearby broke through a police barrier as they stormed the festival, the tabloid Expressen reported.

They tore down festival tents, using tent spikes as weapons against police and throwing stones at officers, the paper said.

Police said at least 52 people had required medical attention, either at the scene or at local clinics and hospitals.

By 7:00 pm, 15 people had been taken to hospital, regional healthcare authority Region Stockholm said in a separate statement. Eight of them had "serious injuries," with the other seven sustaining "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, said.

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.