Lollapalooza turned into Muddypalooza this weekend ... with festivalgoers trading the mosh pit for a mud pit.

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The scene at Chicago's Grant Park looked less like a music festival and more like WrestleMania in a pigpen ... as fans squared up in ankle-deep sludge and threw down for a roaring crowd.

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Two shirtless dudes kicked off the filthy fight card ... grappling and rolling through the muck before a pair of women turned another patch of Grant Park into their own muddy main event during The Neighbourhood's set.

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Heavy rain soaked Grant Park on Friday and Saturday ... leaving behind huge puddles and enough mud to make Woodstock look like a spa day. Saturday's opening was also delayed until 3 PM, with some performances shortened or scrapped.

The skies are expected to stay dry for Sunday's finale ... though the grounds will still be a mess when Tate McRae and The xx close out the festival.

Forget the mosh pit ... the mud pit stole the show!