It is against the law...

[Servin]

... to start a fight within the city and you can get reported and end up punished. That's with weapons.

But what about a bar fight? No weapons involved, fist to face? Those are quite common even amongst the non-adventurer types. Given their nature does (should) not result in death, do the rules consider it as something else?

[Maecius]

It depends on many different variables. Such as the city and tavern in question, the level of violence and destruction, and how long the fighters stick around.

In a highly-policed city, or in an upscale tavern, the guards might be called and you might end up arrested. In other towns, the fight might be over and the fighters dispersed before any guards are notified, if they are notified at all.

It's sort of a case-by-case thing, here.

Starting a bar fight in the Helm and Cloak Inn, in the palace district of Baldur's Gate? Probably will be spending the night in jail.

Starting a bar fight in Beregost right now, with its current political climate, where the guards have more important things to worry about? Probably won't have anyone called on you, unless a templar is nearby and feels like "taking back his town."

Down in the Underdark, if you are in a bar fight against a mage, and you yourself are not a mage, you might end up punished. Even if the mage is the one who started it.

But again, it'll depend on a lot of things, including the individual, responding DM's judgement. Typically a bar fight will only result in a fine or a "night to cool off" in a jail cell, when it is punished, though -- not a more drastic punishment like execution or banishment.

[Hoihe]

Maecius, don't forget the Elfsong Tavern.

So long there isn't killing or large destruction, it's considered the Mos Eisley Cantina of Baldur's Gate. Steve has a post on it somewhere.

[Servin]

(...) the fight might be over and the fighters dispersed before any guards are notified, if they are notified at all.

In most cases the DM response, even if from one online, is - make screenshots - I'm busy in an event.

Hence my question how would one face consequences without the DM witnessing the occurrence would roll out. However, your answer gives a more less of an idea. Thank you.