Barca lost the first El Clasico of the season at the Nou Camp as Real Madrid controlled the match on the day.
Afterwards, there was chaos around Koeman's departure from stadium carpark as fans surrounded his car when he attempted to leave.
There were many circling the car who shouted "out, out", other fans whistled at the Dutchman and one fan was seen attempting to climb onto the car's hood while recording himself on his phone, reports Marca.
Observers noted that there a was a total lack of security inside the carpark as Koeman sought to leave, with dozens of fans clear to approach and surround the Barca coach's car without resistance.
In the aftermath, Barca management released a statement condemning the fiasco.
"FC Barcelona publicly condemn the violent and disdainful acts that our manager experienced when leaving the Camp Nou. The Club will take security and disciplinary measures so that such unfortunate events do not happen again," Barcelona said in a statement.
On Sunday prior to the confrontation, David Alaba put Real Madrid ahead in the first half of the El Clasico and Lucas Vazquez doubled their lead in second-half stoppage time before Barca substitute Sergio Aguero scored for the hosts, as they lost 2-1.
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