A footballer was seriously injured after part of a chair was hurled from the stands holding his own side's fans.
Aberdeen FC player Jack MacKenzie was taken off the pitch in a wheelchair with a bloodied bandage around his head and needed stitches.
Police condemned the "mindless act" of violence which happened at Dundee United's stadium after Aberdeen's 2-1 loss on Saturday.
Dundee fans had invaded the pitch at Tannadice after their side's win, which clinched them fourth place in the Scottish Premiership and a place in Europe.
Amid the chaos, unused substitute MacKenzie was struck by the object and was given lengthy treatment on the pitch.
Aberdeen manager Jimmy Thelin described the incident as "totally unacceptable" while a spokesperson from the Scottish Professional Football League slammed the "moronic behaviour".
A Police Scotland statement read: "Part of a chair was thrown from the upper deck of the Fair Play Stand striking an Aberdeen player. He received treatment at the scene and will require further medical assessment."
Police match commander Superintendent Ray Birnie labelled the incident a "mindless act" and appealed to all fans to help find the person responsible.
Dundee's lap of honour was called off afterwards.
Aberdeen manager Mr Thelin said: "It is not acceptable, it is really, really bad. I haven't seen exactly what happened, but our player got hurt, bad. I think everyone understands that is not OK."
Dundee boss Jim Goodwin also condemned the violence. Goodwin - who previously managed Aberdeen - said it was "an absolute disgrace."
He added: "For him to be hit and seriously damaged by an object thrown by his own supporters, it's crazy."
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A spokesperson for the Scottish Professional Football League said: "It's enormously disappointing and frustrating that one of the most exciting games in the season has been marred by such moronic behaviour.
"We await the match delegate report but will work closely with both clubs and Police Scotland to help identify the perpetrator and fully expect that appropriately severe footballing and judicial sanctions will be imposed."
A statement from Aberdeen said the club was "dismayed and outraged at the scenes" and called for supporters who witnessed the incident to come forward.
"When the perpetrator is identified, we will take the strongest possible action available to us," it read.
(c) Sky News 2025: Footballer hit in the head by seat thrown from stands by own fans
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