Legia were already 2-0 up in the 77th minute on Sunday, at which point the away supporters began throwing flares before invading the pitch.
Riot police were called in to calm the tensions, but the game had to be abandoned nonetheless.
As such, Legia were handed a 3-0 victory in the Ekstraklasa final-day clash - meaning they were crowned champions.
Getting lairy at 3:50
"As a result of this decision, Legia Warsaw has 70 points and the title of Polish champion is assured. Lech Poznan has 60 points and third place on the podium," the Ekstraklasa said in a statement.
"We are not the only stadium where flares have been brought into and interrupted the match," Lech Poznan spokesman Lukasz Borowicz told PAP after the game. "It's too early today to estimate the losses."
Jagiellonia Bialystok won their last match 2-1, but finished second on 67 points.
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