From the point of view of a leftist I would say that it sometimes be justified to use violenc. May it be in cases of self defence or as a last means.

Using violence in order to achieve a given goal must have a solid political basis. Thus before you decide to go over to violent actions you must analyse if this violence is acceptable for a relevant part of the people.

Having said this I do not (I can only speak for Europe) at the moment see any scenario which makes violence acceptable.

Example: Together with my sons I took part in a big Germany wide demonstration against old and new nazis in Dresden. We blocked their intnded march so that they could not even leave the train station. Blocades like that are judged violence by German courts.

During that day several times the police launched provocations, they often do that on anti-right demonstrations. This would never justify violence against the police. The enemy were the fascists, not a police, even when it is confirmed that a relevant part of the police sympathizes with right wing moves.

People in Germany in their majority are anti fascist, but at the same time the use of violence is very unpopular.

Other example: Some decennia ago militant organisations fought against the colonialists in their countries. This was not only a justified struggle, the use of violence was widely accepted by the people of these states.

Personnally I prefer non violent actions such as ghose lead by M. Gandhi. But it needs a very high level of heroidm and self restraint to stand against military knowing that the next moment they will attack you, maybe kill you.

And, coming to the end of this a bit too long posting: My sympathy is strong with the people who fight for their human rights. They themselves have to decide which level of violence they use. And we have to be very careful to analyze wether violence during the actions really came from the protesters or from agents provocateurs (as for example during the riots which aimed to bring people to think all BLM acrivists are looters, rioters etc)

There is a point which I think we are not seeing, and that is very important. What you refer to as Hooligans @Hans are, whether we like it or not, members of our society. Something happened during their upbringing that resulted in them feeling they don't belong in this society, that the public property we all value is for them useless. Something else must have happened in their lives that makes them resort to violence as a way to express the emotions they feel. Most likely they have been victims of violence themselves. Probably physical violence within their families, but also, very likely, social violence.
I want to expand on this last point: What is social violence, as least as I am using the term in this argumentation: Society in developed countries like the Netherlands functions with a high degree of agreement on social conventions. It is agreed by a majority that certain status-quo is good, correct, etc and that deviations from certain expected behaviors or social phenomena is to be frowned upon.
For example: If you are the descendant of a migrant family, you will have less opportunities to get a job. You will most likely live in a neighborhood with people from a similar ethnic background, and you will therefore not go to the same schools as the middle-upper class white children. This, in the case of the Netherlands, will also most likely result in you ending up in a highschool that prepares you for practical work, and not for university education. Regardless of whether this is a good or bad thing (I don't want to imply University is better than technical jobs), it does result in these youth feeling as if their possibilities are limited, feeling like they have less options for their future than other, more privileged members of society. Then it comes time to work, and they might end up making less money than they would like to make. What amount of money would they like to have? Enough to feel like a well-adjusted member of society. And what is a well-adjusted member of society? That depends on what are the role models and archetypes present in mass media and social networks. Have a big house, have a car or a fancy bike, own expensive clothes, have the latest technological whatchamacallit, etc. Is this the right way of considering yourself a functional member of society? I don't think so. But then again, I am not surprised a big part of today's youth has these ideals, because that is what they are fed by mass media and social networks.
So they end up accumulating frustration. Why can't I go to that high school? Why can't I get that type of jobs? Why can't I live in a fancy house in the center of the city? Why can't I be with one of those fancy men/women that are shown on TV? Why do security guards follow me around whenever I go shopping? The list is potentially endless.
And it is no wonder this ends up in anger accumulating towards society. If I can't have it, neither should you. It's unfair, and I'd rather break it so that neither you nor I can have it.
I don't want to claim to knowing what goes through the mind of these, as they have been called here, hooligans or 'violent protestors'. But I wouldn't be surprised if what I have just described is somewhat close to what happens, most likely at an unconscious level.
What does this mean for us? We need to remember that whenever we ostracize people from society, we might pretend they are not there anymore, but they are. Their children will be, and the more they feel like outsiders, the more conflict will arise. This is not the first time violence has taken to the streets in The Netherlands, for seemingly 'no reason'. I think that the reason might not be obvious and direct, but that it might be the results of years, decades of resentment that has been building up from early childhood, and across generations even. This should be a wake-up call for any government. These people are not going anywhere. They are still part of society. We still share a city with them. So we better make sure to eventually integrate them instead of pushing them further away from our sight and our opinion bubbles.

@Bosshammer I referred to families of different ethnicity because in the Netherlands there is a big social divide between the local white population and the migrant working class population. A divide that is expressed in the jobs they get, but also the places they live in the city and the schools their children go to. But yes, you have a point. This applies to all the working class. In fact, with the arrival of highly educated migrants to the Netherlands, more and more 'originally' Dutch working class is entering into the group of 'the ones left behind'.

I agree with you that the police can be very violent. When we occupied the University of Amsterdam a few years ago an anti-riot police crashed me against a wall using a horse, and I had done nothing more than just stand there and protest. I saw them using violence against people who had not committed any violence against anyone. Same goes for the protesters of Extinction Rebellion when they stop traffic. You won't see me defending the police.

It is worth noting, however, that the rioters in Rotterdam did attack the police with fireworks.

In any case, my original post was directed at the people protesting against covid regulations. Since the topic shifted towards looters, I wrote something about that group as well. But I see them as different groups, although you can see them both in action in these anti-covid protests going on today.