In Sweden, nearly 100 men gathered Friday night in the capital, handing out leaflets calling for attacks on migrants, Stockholm police spokesman Albin Naverberg told CNN.
Some of the men were wearing black clothes and masks, he said.
Reports of attacks on people from immigrant backgrounds surfaced in Swedish media and on social media, but police said they had received no reports of assaults on civilians.
One man was arrested in an attack on a plainclothes police officer, although the officer was not seriously injured, and five others were detained for disorderly conduct, Naverberg said. All six were released Saturday morning.
Police had known of the gathering in advance and had deployed a large presence, making it difficult for the group to carry out attacks, Naverberg said.
"There were many police officers at the place, and it was hard for them to do that," he said.
Protesters turned out Saturday in central Stockholm to demonstrate against the actions of the mob the previous night.
Friday's tensions were the latest in a series of incidents involving migrants in Sweden. Last week, a 15-year-old asylum-seeker fatally stabbed a refugee center worker in western Sweden's Gothenburg region
, police said. The victim was a 22-year-old woman, who was alone with eight young male residents at a home for asylum-seekers.
Far-right, anti-racism protesters face off in UK
And on Saturday, tensions over the migration crisis spilled over to the UK, reflecting growing anti-immigrant sentiment across Europe as countries struggle to respond to the wave of migration from the Middle East and Africa. More than 1 million migrants entered the continent irregularly last year
In Dover, where ferries depart for France and near an underwater tunnel connecting the UK with continental Europe, violent clashes erupted Saturday as far-right and anti-racist groups faced off in rival demonstrations.
Right-wing protesters carried signs stating, "Multiculturalism has failed," while rival demonstrators carried placards saying, "Refugees welcome here."
Kent police said that one person suffered a broken arm and five others sustained minor injuries, and three people were arrested.
More than 20 weapons were seized, including a knife, brass knuckles, poles and pieces of wood, hammers and bricks, police said.
Footage of the clashes, posted on the social media accounts of far-right group the South East Alliance and anti-racist movement the Kent Anti-Racism Network, showed violent street fights between rival protesters, some of whom were wearing masks.
Police in riot gear used batons and smoke bombs to try to control the groups.
Earlier, six people were arrested on suspicion of violent disorder at a motorway service station near the town of Maidstone, police said, following a clash between rival groups en route to the demonstrations.
A bus carrying a group of anti-racism demonstrators had a swastika daubed on it in blood, according to a social media post by a passenger.