Berlin - Hardcore Borussia Dortmund fans attacked terrified RB Leipzig supporters, including children and women, in football battles that led to 28 arrests, German authorities said.
A crowd of 81,360 saw Dortmund's 1-0 home win over second-placed RB Leipzig on Saturday, but there were ugly scenes outside Borussia's Signal Iduna Park stadium before kick-off when some of the 8,500 visiting fans were targeted.
"In the build-up to the match, in the Strobelallee area (outside the ground), RB Leipzig supporters were bombarded with stones and cans from Borussia Dortmund fans," police said in a statement.
RB Leipzig were unable to tell SID, an AFP subsidiary, how many of their fans were hurt, but German daily Bild claim at least 10 visiting supporters were hospitalised.
After a group of around 500 hardcore 'Ultra' Dortmund fans went on the rampage, police weighed in with pepper spray and batons.
Arrests were made for offences including assault, grievous bodily harm, breach of the peace, insulting behaviour and resisting arrest.
"All in all, there was extreme aggressiveness and violence from Dortmund supporters, which was directed against any person recognised as a Leipzig fan, regardless of whether they were small children, women or families," said Dortmund police.
"In connection with this, four officers and a police dog were injured."
Borussia Dortmund said they "deeply regretted the riots" and wished the injured Leipzig fans a speedy recovery.
Leipzig, who are in their first season in Germany's top flight, have demanded an explanation from Dortmund's CEO Hans-Joachim Watzke and president Reinhard Rauball.
"The attacks of Dortmund fans against opposing spectators and the police, plus insults and offenses against women and children, are not acceptable and are embarrassing for the whole of football in Germany," said RB Leipzig in a statement.
"The fact that people were confronted with violence at a football match is unacceptable.
"We expect from Mr Watzke and Mr Rauball that these scenes, which were committed by several perpetrators, will be fully explained in the interests of the entire Bundesliga.
"We are glad that, according to our information, the injured RB Leipzig fans were able to return home."
RB Leipzig, who are backed by energy drinks giant Red Bull, are unpopular amongst German fans for the commercialism the club is perceived to represent, but Dortmund fans were particularly unwelcoming.
There were several offensive banners in the Dortmund stadium aimed at RB, including "Burnout Ralph, go hang yourself!" after Leipzig's sports director Ralf Rangnick resigned from Schalke in 2011 with burnout.