When I happened to glance up the street, among the passersby I saw one person who had stopped and was looking down at me.
Literal
Suddenly street [to-に] eye [obj-を] direct-[when-と], go-and-come people [gen-の] middle [in-に], 1-person stop-still me [obj-を] look-down person [subj-が] existed.
A dense narrative sentence featuring several characteristic devices. ふと 'suddenly / on a whim' is a set adverb for unplanned action. 目をやる (literally 'to direct one's eyes') is an idiomatic set phrase for 'to glance at.' と here is the conditional marking discovery — 'and when I did X, it came into view that Y.' 行き交う is a compound verb (行く + 交う) meaning 'to come and go / pass each other.' 立ち止まり is the conjunctive masu-stem of 立ち止まる, chained without ~て in written style. 見下ろす者 is a relative clause modifying 者 'person,' with 者 chosen over 人 for a slightly literary tone.