When was the first time in recorded history where the juridical system provided for protecting the rights of the general population, including the lower classes, so that a person of lower class could have a realistic chance to win a trial / legal battle, even against an opponent of higher social status.
To make this question answerable, I'll include a list of constraints.
it doesn't have to be a perfectly equal or perfectly fair chance. Even today, even in the most egalitarian countries, someone who can afford a better lawyer or knows a lot of important people has at least somewhat increased chances. The point is having any realistic chance at all.
if it was only de jure, but never practiced, it doesn't count. For it to count, there have to be at least some recorded cases of normal trials where it happened.
Special occasions (the king on his birthday or on a national holiday providing justice for a poor beggar, or otherwise a highly notorious and very individual case) don't count if those people couldn't have had any chance at all at a regular trial.
A formerly oppressed group gaining power in a revolution and therefore securing the rights doesn't count, unless they provide fair trial for the newly subjugated class. (For the sake of this question, the winners of the revolution no longer count as the lower class)
Note: if the underlying societies would prove to be so different that the very terms of "trial" , "law", or "lower class" would have vastly different meanings to decide a "winner", one could list other, possibly later cases from a different continent / completely different civilization.