A gang of up to 100 masked men dressed in black attacked migrants in central Stockholm Friday evening, the BBC reports.

Witnesses said the men physically attacked people thought to be foreigners. One man punched an officer in the face and was arrested, according to a statement released by Swedish police.

The Swedish Aftonbladet newspaper reported men also distributed leaflets on Friday evening that contained the slogan “It’s enough now!” The leaflets threatened “North African children who are roaming around” with the “punishment they deserve.”

A 16-year-old told Aftonbladet he was hit in the face near Stockholm’s central station. Another witness said he saw men beating up people who looked like foreigners in Stockholm’s Sergelstorg Square.

Swedish police said it is unclear whether the masked men belonged to a certain group. The men were wearing colored arm bands and reportedly have links to football hooligan gangs.

Sweden took in more than 160,000 asylum seekers in 2015. It accepted the highest number of asylum seekers per capita in Europe.

Tensions over the migrant crisis in Sweden heightened on Monday when a 15-year-old asylum seeker stabbed to death a 22-year-old Swedish woman. The murder occurred at a home for asylum seekers under the age of 18, and the victim was an employee of the refugee center.

On Thursday, Swedish Interior Minister Anders Ygeman said the country is preparing to deport up to 80,000 migrants whose asylum requests were denied.