Mob calls for assaults on foreigners in Sweden; clashes at UK migrant protests

London (CNN)Tensions over Europe's migrant crisis led to arrests and violence in Sweden and the UK over the weekend, with a masked mob calling for attacks on foreigners in Stockholm and clashes between pro- and anti-migrant demonstrators in the British town of Dover.

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.

Sweden's interior minister said last week that the country could expel up to 80,000 of the 163,000 people who sought asylum in the country last year.