BERLIN (AP) — Fan trouble and the threat of violence are overshadowing Friday's start to the Bundesliga.

Defending champion Bayern Munich hosts Bayer Leverkusen to get the league underway, but unsavory scenes in the German Cup and the threat of more to come have dominated headlines in the build-up.

"I have become very concerned in the last weeks and months that there were martial marches in the context of football games, 'war declarations' and inhuman actions against teams and their fans," German soccer federation president Reinhard Grindel said Wednesday. "Football can't stand for that. It has to stop."

The latest incidents occurred on Monday, when Hertha Berlin's game at Hansa Rostock was twice held up. First, Hertha fans lit flares and fireworks in their corner. Later, the match was stopped for a much longer suspension when Rostock fans taunted their rivals with a Hertha banner stolen from a previous game. They set it alight as fireworks flew in both directions. Some landed on the field, and referee Robert Hartmann led the players off as fires broke out amid unoccupied seats with riot police lined up alongside.

The police were unable to intervene because of a fence separating them from the masked fans, who continued taunting their rivals as others joined in derogatory chants. The game was not abandoned, but resumed more than a quarter of an hour later to the sound of explosions.

"That will occupy everyone in the coming days and weeks - federations, clubs, fans. It absolutely cannot go on like this," Hertha general manager Michael Preetz said.

Officials were helpless to prevent the trouble despite it being labeled a high-risk game.

"When you see that there were 1,700 police and more than 300 wardens here, that sniffer dogs and high-definition cameras are in operation. Everything that can be done to keep order was done," Rostock chairman Robert Marien said. "It can only be solved by society as a whole."

Rostock police chief Michael Ebert said it was likely club officials had facilitated the stolen banner's entry into the stadium, a charge rejected by Marien.

Potential trouble was averted in Berlin earlier Monday when police detained 91 Dynamo Berlin fans before their side's game against Schalke. They had received information that rival fans had pre-arranged a fight. The police, who found balaclavas and expandable batons among the fans, released the detainees after the game.

Some ultra groups previously displayed banners calling for "war" against the German federation in response to a general clampdown on pyrotechnics at games.

Despite being banned, pyrotechnics featured prominently at many games last season, including the German Cup final between Borussia Dortmund and Eintracht Frankfurt. Security had been heightened for that game, but still fans managed to smuggle in their flares and fireworks.

Federation general secretary Friedrich Curtius said this showed "the high criminal energy" of some fan groups.

Bavarian interior minister Joachim Herrmann has called for clubs to distance themselves from the ultra fan groups.

"The statements that I've been made aware of from the ultra scene in the last days are really totally inacceptable," Herrmann told TV station Sat. 1.

Other politicians have echoed his call for tougher sanctions and zero-tolerance for violence, as have club officials.

"We're on the wrong path when we're taking about a war-scenario in connection with football," Preetz said. "We have to have a factual discussion. That will be hard enough in the next weeks."

Grindel called Wednesday for a change in how the federation deals with infringements, to refrain from collective punishments such as fines for clubs and partial stadium closures - these were the standard responses before.

"We have to build trust through dialogue," said Grindel, who said the federation very much welcomed talks with ultras in Dresden. "We want to find common ways to achieve transparent and fair measures to ensure a positive stadium experience."

Grindel referred to the positive sides of German soccer culture - the impressive choreographies, secondary events outside stadiums, campaigns for tolerance, low ticket prices, the great atmosphere at games - but said clear lines have to be drawn, including the "the renouncement of violence."

"It's time for a rethink," Grindel said.