Police said that one attack in the city centre committed on Saturday by people armed with a telescopic baton and a knife was carried out by a 19-year-old, a 20-year-old and a 23-year-old who were identified through a large number of witness reports.
A second attack north of the city centre took place on Sunday after a group of six attackers approached four young men aged 20-22 asking about their nationality.
Once they had confirmed that their victims weren't German, they attacked them with bottles, wooden slats and a baseball bat, the Westdeutsche Allgemeine Zeitung reported.
Victims told police that they were also kicked once they had been beaten to the ground.
Although the attackers initially escaped, the four victims called police an hour later to say they were hiding from the group again.
Officers arrested four of the men, including one who threw a knife away into bushes before he was caught.
Police said the four men were aged between 19 and 51 and had strong connections to the local neo-Nazi scene.
A special far-right extremism police unit has taken over the investigation.