Story highlights Mob hands out leaflets in Stockholm calling for assaults on migrants, police say

In UK, pro- and anti-migrant protesters clash in Dover

Tensions rise as Europe faces largest migrant crisis in decades

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.