Liverpool manager Jurgen Klopp felt some protestors crossed a line during the demonstrations that caused their match with Manchester United at Old Trafford to be postponed.
United supporters opposed to the Glazer family's ownership of the club gathered at the stadium and outside the hotel where the teams were staying ahead of last Sunday's match as part of planned attempts to disrupt the fixture.
Some fans were able to get onto the pitch at Old Trafford and there were some violent clashes with police outside the ground.
United issued a statement saying a minority of fans had caused criminal damage to access the pitch, while Greater Manchester Police said 12 officers had reported injuries, one of whom had a broken eye socket and facial wound.
One man has been charged with throwing fireworks in a street, using threatening behaviour and wilful obstruction of the highway for his part in the protests at the Lowry Hotel.
Klopp, who was watching TV coverage unfold with his players in the hotel lobby as he waited to learn whether the game would go ahead, said such protests should always remain "completely peaceful".
Speaking ahead of his side's Premier League match with Southampton on Saturday, he said: "Am I surprised that these kinds of things happen? No. I'm an absolute believer in democracy, [which] means I'm happy that people want to tell their opinion, that's completely fine, but I know that in situations like this it doesn't happen often that nobody gets hurt.
"That's why I said a week before when all the pundits were going and asking for protests and stuff that we had to be careful and calm down and ask for the right things. I heard some policemen got injured and these kinds of things should not happen, breaking down the doors and stuff like this.
"To tell your opinion, to tell the world what you think about some things is completely fine, as long as it's completely peaceful. It was not, as far as I know, completely peaceful. Then, it's not right any more."
The United match was rearranged for May 13, a decision that left Ole Gunnar Solskjaer angry as it means the Red Devils must play three times in just five days.
"Of course, I feel sympathy for him because it's not how it should be, but it's not good for us, to be honest. I would have preferred to play the game last week," Klopp said.
"Now, we have five games in the last two weeks, so that's not really cool as well, but it is how it is. Nothing to complain about or whatever. It's just the situation."
Liverpool will see Manchester City regain the Premier League title if they defeat Chelsea on Saturday, while the Reds are themselves embroiled in a battle for Europe. They sit seventh in the table, seven points outside the Champions League places, with five games to go.
Klopp, who is not giving up on breaking back into the top four, thinks he and his players will have learned a lot from a difficult season blighted by injuries.
"I know people think that's an excuse," he said. "In the moment when we lost our full defence, it felt like you break your leg. That's okay, you can still limp with that, but then we had to make our midfielders to transform them into defenders. Then, we broke pretty much our spine, and it's difficult to move.
"In the beginning when we had problems, we were still kind of winning, in a good position but it's a long season and if anything else happens, we will struggle, and that's what happened.
"To be in the position we were two years before, we needed to be nearly perfect and the situation needed to be nearly perfect. In a moment when it's not perfect for us, the league is too strong.
"Could we now, instead of seventh, be fifth or fourth? Yes, of course, no doubt about that. We should have done better in specific moments. But you asked me what was the challenge: to deal with the amount of injuries was the challenge, and we couldn't deal with it as well as we wanted throughout the whole season.
"I was most of the time not champion in my career, so I try to learn always. Did I learn a lot? Yeah, for sure. Some things you want to learn, some things you don't want to experience.
"The players learned a lot as well – that's clear. That's the plan: that we use our new experience for the future, for next season especially, but you'll have to wait until then so we can show that we really got it."