The German Football Association (DFB) has opened an investigation into the actions of Borussia Dortmund and Hertha Berlin supporters following incidents that occurred in the away end during the teams' 2-2 draw in Dortmund on Saturday.
Dortmund police said in a statement that 45 people were injured, 35 of them after the use of pepper spray inside the Westfalenstadion, and a further 10 "because of acts of violence." Hertha condemned the incidents and independent fan organisations criticised the "excessive" police operation.
In a statement, the DFB said on Monday that they have opened investigations against both Bundesliga clubs, who will be requested to comment on the incidents.
"After a review of the statements as well as other material such as TV and video footage and the security statements, the DFB control body will decide about the progress of the proceedings," the DFB said.
Violence broke out in the away end at the Westfalenstadion during the opening minutes of the Bundesliga match when police tried to grab a Hertha banner. Before kickoff, the banner had been used as part of a tifo celebrating the 15th birthday of Hertha ultra group Hauptstadtmafia, or Capital Mafia.
Hertha supporters also used the banner as a shield from cameras and set off flares, and police said they wanted to "seize the banner to prevent further offences."
In German stadiums, the use of flares and setting off pyrotechnics is illegal. However, police have not intervened in recent years and instead have used video technology to identify the perpetrators.
A banner is considered sacred to German ultra groups, with some in the past disbanding when the banner was stolen or lost.
On Saturday, Hertha supporters jumped over the advertising board into a space between the away end and the pitch. They defended their banner by attacking police with flagpoles, and video footage appearing on social media also showed one flare being thrown at the police.
Other Hertha supporters defended their banner from above the ad board.
"This behaviour was prevented through the use of pepper spray and riot sticks," police said.
At half-time, more violence occurred underneath the away end. Hertha supporters destroyed the restrooms and attacked police with the destroyed leftovers.
Sources have told ESPN FC that fewer than 10 Hertha supporters remained in Dortmund's hospitals as they received treatment following the use of riot sticks and pepper spray.
In a statement released on Saturday, Hertha condemned their supporters' behaviour and said they would do all they can to identify those setting off pyrotechnics. The club also said they will "analyse the reasons for the police operation."
In a joint statement, Hertha's and Dortmund's independent fan aid agencies criticised Dortmund police.
"The use of pepper spray in a packed fan block can't only be restricted to alleged disturbers," they said. "It instead hits every fan close to scene. Those excessive measures could have led to mass panic. That's why police operations are not standard in other stadiums while in Dortmund, long and dangerous clashes were provoked."