I'm not 100% sure exactly what meaning and tense you are trying to convey.
I may have to complete your sentence fully to show you what I mean.
If you just want to convey the fact that there are lots of dead (or whatever) folks in the street:
Present tense narration:
"She has to step over and around the thousands that lie in the middle of the street, filling the city."
or:
Past tense narration:
"She had to step over and around the thousands that lay in the middle of the street, filling the city."
(Note that in these cases I have removed "simply" and "down" as not needed.)
If you want to tell us what the people did before the character arrives:
"She had to step over and around the thousands that had simply laid themselves in the middle of the street, filling the city with their dead bodies."
(Note that you need the object "themselves" to go with the verb "laid" in this case. If you want a verb without an object, I would change it to something like "simply collapsed in the middle of the street.")