I am currently on the Poker Face: Loc Muinne-quest, and I must go across a marketplace in Loc Muinne in order to meet the final opponent in Dice Poker, however, there is a two-digited number of Temerian guards on the marketplace, and it does not seem possible to sneak past them. I have tried fighting them, but it is very difficult, as I am playing on dark difficulty, and I do not want to lower it now. I'm guessing I also need to go through this route in order to progess in the main storyline. Is there a different option to killing all the guards, or should I just steel myself and do it?

Do you have dark mode armor and weapons?

What do you mean? I have an armor with 24 armor bonus, and a steel sword with 40-44 damage.

There are sets of armor and weapons which are very powerful and only available for dark and insane difficulty modes. Those are way more powerful and make life/not dying in-game easier.
Look through the link I mentioned below to get an idea of how powerful the equipment is.

http://nogamenotalk.com/2011/10/07/your-more-or-less-definitive-guide-to-geralt%E2%80%99s-dark-armor/

Nice! I did not know about this, I will get these the next time I play through the game. Unfortunately for my current playthrough though, I only have a third of the orens I need.

I may be wrong here but the area you mentioned seems like the Temerian camp where you can choose to get arrested to see Phillipa whereas Random_Coffee is talking about the market area of Loc Muinne (Isn't that the area with blacksmith, Temporary Inn,etc.) .

I'm pretty sure you can get arrested by any guard, and the only place in Loc Muinne I can remember that has double-digit Temerian enemies is this place:

https://www.youtube.com/watch?v=eDw7jKnaG-A

... which, in addition to being 100% unnecessary, isn't smart to fight through unless you have an insanely powerful build. Besides, I seem to remember the majority of chapter 3's dice quest being inside that little building that you find when you enter the city and stick to the rightmost path. I can imagine OP doing dice poker right away and then trying to follow the quest marker through the Temerian quarter without realizing that there's a ramp that goes around just outside of the dice poker building they were just in.

The area with the blacksmith and all that jazz is in the market where The Incredible Lockheart (or whatever) is, which means that's where OP is trying to go rather than where they're starting from. The marketplace itself only has a few soldiers, and they don't attack you unless provoked.

I went up the ramp 227 mentioned and found the marketplace I needed to go to. Thanks for the help :)

I also just realized that the place with all the Temerian guards is not a marketplace, but I thought it was at first, because there were tents and stuff there. My mistake.

And also, kudos to you firedash for letting me know about the dark gear :)