The singer was tapped for the opening ceremony show at the Stade de France in Saint-Denis just north of Paris -- where Real Madrid was facing off against Liverpool in a huge showdown between Europe's best soccer teams ... a must-see match fans were dying to see.

That might even be literal, because tons of prospective gamers were outside the stadium shortly before kickoff -- while Camila was doing her gig -- and desperately trying to get in by any means necessary ... with several of them hopping gates and having run-ins with security.

Some of these videos are absolutely insane -- people are climbing over each other and risking it all just for a chance to get inside. Unclear if these are people who purchased tickets and were turned away, or if they're simply hooligans trying to take it in for free.

At any rate, the cops and security officers on hand weren't having any of it ... pepper spraying people who were trying to rush the gates and who wouldn't back down. There are some reports (and videos) of minors getting tear gassed as well.

Even the cops themselves were trying to wipe the residue from their eyes ... that's how much they fired off, it seems.

As for Camila ... it appears she was completely oblivious to the pandemonium happening just a couple hundred feet from her on the pitch -- and paid it no mind, doing what she was there to do ... sing. Even if she knew what was going on, it's not really her problem.

BTW, she killed it during the show ... performing for a packed stadium and putting on an elaborate spectacle with backup dancers and a grand entrance to set everything off.

While no one is faulting her for continuing on with the ceremony, some are pointing out it's funny that the contrast here is so stark -- and frankly, it is. A lesson learned ... get to your seats early and often. Also, Euro soccer fans are low-key bonkers for the beautiful game.