Bayern Munich's game against Bochum, a 3-2 loss, was briefly suspended due to fan protests against proposed private equity investment as they fell further behind league leaders Bayer Leverkusen.
Bochum fans threw the tennis balls onto the field, forcing an early halt to play. As soon as they were cleared, Bayern Munich supporters flung more tennis balls from their section, leading referee Daniel Schlager to tell both teams to return to their dressing rooms. The fans also chanted against the German football league (DFL) with an expletive.
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The match resumed some 13 minutes later. Fans across Germany have been protesting at games against a proposed deal to sell a future stake in the league to a private equity investor.
In previous protests, apples and chocolate coins have been thrown onto the pitch. On Friday, remote control cars were sent careening around the penalty area during Werder Bremen's 1-0 win at FC Cologne, while last week, a game between Hamburg and Hannover in the second tier was interrupted after fans attached bicycle padlocks to a goalpost. Officials cut the locks off with a power saw.
Bayern Munich's defeat puts pressure on manager Thomas Tuchel as the team's quest for a 12th consecutive league title grows increasingly unlikely. Bayern Munich are now eight points behind Xabi Alonso's unbeaten Leverkusen side.
Bayern took the lead in the 14th minute as Germany international Jamal Musiala found the net from a tight angle, before Bochum began to turn the tide.
Takuma Asano equalised before Kevin Schlotterbeck made it 2-1 with a towering header before half-time.
A penalty kick from Kevin Stöger made it 3-1 in the 85th minute, making Harry Kane's 94th minute tap-in nothing more than a consolation.
Information from the Associated Press contributed to this report
